Diamond/Platinum open access journals are those journals which do not collect Article Processing Charges (APC) from their authors. While it is impossible to list all the journals, The Directory of Open Access Journals (DOAJ) is a helpful entity in the search. It is an online directory that indexes and provides access to high quality, open access, peer-reviewed journals. All data is freely available.
Disclaimer: I cannot vouch for the quality of the listed journals and authors are requested to submit a manuscript only after due research about the journal. While these journals are filtered for no APC, kindly make it sure with the journal before submission to avoid any hazzles.
So here is a list of journals coming under the purview of mathematical science, hope you find this useful.
1. Journal of Numerical Cognition, ISSN: 2363-8761 (Online)
Journal of Numerical Cognition
2. Journal of Algebra and Related Topics - JART, ISSN: 2345-3931 (Print); 2382-9877 (Online)
3. Arabian Journal of Mathematics, ISSN: 2193-5343 (Print); 2193-5351 (Online)
Arabian Journal of Mathematics
4. E-Jurnal Matematika, ISSN: 2303-1751 (Print)
http://ojs.unud.ac.id/index.php/mtk
5. Modern Stochastics: Theory and Applications - MSTA, ISSN: 2351-6046 (Print); 2351-6054 (Online)
Modern Stochastics: Theory and Applications
6. Scientia Magna, ISSN: 1556-6706 (Print)
7. Integers - Integers: Electronic Journal of Combinatorial Number Theory, ISSN: 1553-1732 (Online)
8. Transactions of A. Razmadze Mathematical Institute, ISSN: 2346-8092 (Print)
Shop and Discover over 51,000 Books and Journals
9. Biomath, ISSN: 1314-684X (Print); 1314-7218 (Online)
http://www.biomathforum.org/biomath/index.php/biomath
10. Communications in Combinatorics and Optimization, ISSN: 2538-2128 (Print); 2538-2136 (Online)
Communications in Combinatorics and Optimization
11. Journal of Optimization, Differential Equations and Their Applications, ISSN: 2617-0108 (Print); 2663-6824 (Online)
Журнал з оптимізації, диференціальних рівнянь та їх застосувань
12. Journal of Mathematics Education at Teachers College, ISSN: 2156-1397 (Print); 2156-1400 (Online)
Journal of Mathematics Education at Teachers College
13. Surveys in Mathematics and its Applications, ISSN: 1843-7265 (Print); 1842-6298 (Online)
Surveys in Mathematics and its Applications
14. Mathematical Communications, ISSN: 1331-0623 (Print); 1848-8013 (Online)
15. Journal of the Egyptian Mathematical Society, ISSN: 1110-256X (Print); 2090-9128 (Online)
Journal of the Egyptian Mathematical Society
16. Communication in Biomathematical Sciences, ISSN: 2549-2896 (Online)
Communication in Biomathematical Sciences
17. Mathematica Bohemica, ISSN: 0862-7959 (Print); 2464-7136 (Online)
18. Rendiconti di Matematica e delle Sue Applicazioni, ISSN: 1120-7183 (Print); 2532-3350 (Online)
Rendiconti di Matematica di Sapienza Università di Roma
19. Moroccan Journal of Pure and Applied Analysis, ISSN: 2605-6364 (Print); 2351-8227 (Online)
https://content.sciendo.com/view/journals/mjpaa/mjpaa-overview.xml
20. TWMS Journal of Applied and Engineering Mathematics, ISSN: 2146-1147 (Online)
21. TEMA -Tendências em Matemática Aplicada e Computacional, ISSN: 1677-1966 (Print); 2179-8451 (Online)
22. Journal of Humanistic Mathematics, ISSN: 2159-8118 (Online)
Journal of Humanistic Mathematics
23. Documenta Mathematica, ISSN: 1431-0635 (Print); 1431-0643 (Online)
24. Journal of Honai Math - JHM, ISSN: 2615-2185 (Print); 2615-2193 (Online)
25. Open Journal of Mathematical Sciences - OMS, ISSN: 2616-4906 (Print); 2523-0212 (Online)
Open Journal of Mathematical Science (OMS)
26. Neutrosophic Sets and Systems, ISSN: 2331-6055 (Print); 2331-608X (Online)
27. Problemy Analiza -Issues of Analysis, ISSN: 2306-3424 (Print); 2306-3432 (Online)
Problemy Analiza ○ Issues of Analysis
28. Journal of Mathematics Education at Teachers College -JMETC, ISSN: 2156-1397 (Print); 2156-1400 (Online)
Journal of Mathematics Education at Teachers College
29. Discussiones Mathematicae - General Algebra and Applications, ISSN: 1509-9415 (Print); 2084-0373 (Online)
https://www.degruyter.com/view/j/dmgaa
30. Applied Sciences (Bucureşti), ISSN: 1454-5101 (Online)
31. Exatas Online -Revista Científica do Departamento Química Exatas, ISSN: 2178-0471 (Online)
32. Communications in Mathematics, ISSN: 2336-1298 (Online)
https://www.degruyter.com/view/j/cm
33. Revista de Métodos Cuantitativos para la Economía y la Empresa -Journal of Quantitative Methods for Economics and Business Administration, ISSN: 1886-516X (Online)
Revista de Métodos Cuantitativos para la Economía y la Empresa
34. Balkan Society of Geometers Proceedings - BSG Proceedings, ISSN: 1843-2654 (Print); 1843-2859 (Online)
35. Matrix Science Mathematic, ISSN: 2521-0831 (Print); 2521-084X (Online)
https://www.zibelinepub.com/index.php/matrix-science-mathematic-msmk/
36. Review of Science, Mathematics and ICT Education, ISSN: 1791-261X (Print); 1792-3999 (Online)
https://pasithee.library.upatras.gr/review/article/view/2801
37. Mathematics and its Applications: Annals of the Academy of Romanian Scientists, ISSN: 2066-5997 (Print); 2066-6594 (Online)
http://www.mathematics-and-its-applications.com
38. Theory and Applications of Graphs, ISSN: 2470-9859 (Online)
Theory and Applications of Graphs
39. Mathematics in Education, Research and Applications - MERAA, ISSN: 2453-6881 (Online)
Mathematics in Education, Research and Applications (MERAA) :: ISSN 1336
40. Analele Stiintifice ale Universitatii Ovidius Constanta: Seria Matematica, ISSN: 1224-1784 (Print); 1844-0835 (Online)
Analele stiintifice ale Universitatii Ovidius Constanta, SERIA MATEMATICA ISSN 1224
41. Mathematical Modelling and Analysis - MMA, ISSN: 1392-6292 (Print); 1648-3510 (Online)
Mathematical Modelling and Analysis
42. Categories and General Algebraic Structures with Applications - CGASA, ISSN: 2345-5853 (Print); 2345-5861 (Online)
43. AKCE International Journal of Graphs and Combinatorics, ISSN: 0972-8600 (Print)
http://www.journals.elsevier.com/akce-international-journal-of-graphs-and-combinatorics
44. Boletim da Sociedade Paranaense de Matemática, ISSN: 0037-8712 (Print); 2175-1188 (Online)
http://www.periodicos.uem.br/ojs/index.php/BSocParanMat/index
45. International Journal of Mathematical, Engineering and Management Sciences, ISSN: 2455-7749 (Online)
46. Matematički Vesnik, ISSN: 0025-5165 (Print); 2406-0682 (Online)
47. Topological Algebra and its Applications, ISSN: 2299-3231 (Online)
http://www.degruyter.com/view/j/taa
48. Connections, ISSN: 0226-1766 (Print)
49. Journal of Applied Mathematics, Statistics and Informatics, ISSN: 1339-0015 (Online)
http://www.degruyter.com/view/j/jamsi
50. Dolomites Research Notes on Approximation - DRNA, ISSN: 2035-6803 (Online)
Dolomites Research Notes on Approximation
51. Discrete Analysis, ISSN: 2397-3129 (Online)
52. Journal of Mathematical Extension, ISSN: 1735-8299 (Print)
53. Bulletin of Mathematical Sciences, ISSN: 1664-3607 (Print); 1664-3615 (Online)
https://www.worldscientific.com/worldscinet/bms
54. Épijournal de Géométrie Algébrique, ISSN: 2491-6765 (Online)
https://epiga.episciences.org/
55. Journal of Logic and Analysis, ISSN: 1759-9008 (Online)
56. Annales Academiae Scientiarum Fennicae: Mathematica, ISSN: 1239-629X (Print); 1798-2383 (Online)
http://www.acadsci.fi/mathematica/
57. JPMI (Jurnal Pendidikan Matematika Indonesia), ISSN: 2477-8443 (Online)
http://journal.stkipsingkawang.ac.id/index.php/JPMI
58. Archivum Mathematicum, ISSN: 0044-8753 (Print); 1212-5059 (Online)
http://www.emis.de/journals/AM/
59. JOHME: Journal of Holistic Mathematics Education, ISSN: 2598-6759 (Online)
https://ojs.uph.edu/index.php/JOHME
60. International Journal of Group Theory, ISSN: 2251-7650 (Print); 2251-7669 (Online)
61. Journal of Algebra Combinatorics Discrete Structures and Applications, ISSN: 2148-838X (Online)
http://jacodesmath.com/index.php/jacodesmath
62. Journal of Urban Mathematics Education - JUME, ISSN: 2151-2612 (Online)
https://jume-ojs-tamu.tdl.org/jume/index.php/JUME/index
63. Formalized Mathematics, ISSN: 1426-2630 (Print); 1898-9934 (Online)
http://www.degruyter.com/view/j/forma
64. Journal of Multidisciplinary Modeling and Optimization, ISSN: 2645-923X (Online)
65. Symmetry, Integrability and Geometry: Methods and Applications - SIGMA, ISSN: 1815-0659 (Online)
http://www.emis.de/journals/SIGMA/
66. Journal of Integrable Systems, ISSN: 2058-5985 (Online)
Journal of Integrable Systems | Oxford Academic
67. Karpatsʹkì Matematičnì Publìkacìï - Carpathian Mathematical Publications, ISSN: 2075-9827 (Print); 2313-0210 (Online)
http://journals.pnu.edu.ua/index.php/cmp
68. Communication in Mathematical Modeling and Applications - CMMA, ISSN: 2528-830X (Online)
69. Sukkur IBA Journal of Computing and Mathematical Sciences, ISSN: 2520-0755 (Print); 2522-3003 (Online)
http://journal.iba-suk.edu.pk:8089/SIBAJournals/index.php/sjcms/index
70. Annals of the West University of Timisoara: Mathematics and Computer Science, ISSN: 1841-3307 (Online)
http://www.degruyter.com/view/j/awutm
71. Armenian Journal of Mathematics, ISSN: 1829-1163 (Online)
http://armjmath.sci.am/index.php/ajm
72. Data Envelopment Analysis and Decision Science, ISSN: 2195-4496 (Print)
73. Infinity- Jurnal Infinity, ISSN: 2089-6867 (Print); 2460-9285 (Online)
http://e-journal.stkipsiliwangi.ac.id/index.php/infinity
74. ESAIM: Proceedings and Surveys, ISSN: 1270-900X (Print)
75. Journal de l'École polytechnique. Mathématiques, ISSN: 2429-7100 (Print); 2270-518X (Online)
76. International Journal of Mathematical Modelling & Computations, ISSN: 2228-6225 (Print); 2228-6233 (Online)
http://www.ijm2c.ir/index.php/ijm2c
77. Pesquisa Operacional, ISSN: 0101-7438 (Print); 1678-5142 (Online)
http://www.scielo.br/scielo.php/script_sci_serial/pid_0101-7438/lng_en/nrm_iso
78. Sahand Communications in Mathematical Analysis, ISSN: 2322-5807 (Print); 2423-3900 (Online)
79. Indonesian Journal of Combinatorics, ISSN: 2541-2205 (Online)
80. Transactions on Combinatorics, ISSN: 2251-8657 (Print); 2251-8665 (Online)
81. MEJ (Mathematics Education Journal), ISSN: 2579-5260 (Print); 2579-5724 (Online)
http://ejournal.umm.ac.id/index.php/MEJ/index
82. Annales Mathematicae Silesianae, ISSN: 2391-4238 (Online)
https://www.degruyter.com/view/j/amsil
83. The Teaching of Mathematics, ISSN: 1451-4966 (Print); 2406-1077 (Online)
84. Advances in Decision Sciences, ISSN: 2090-3359 (Print); 2090-3367 (Online)
http://journal.asia.edu.tw/ADS/
85. Bulletin of Computational Applied Mathematics, ISSN: 2244-8659 (Online)
86. Extracta Mathematicae, ISSN: 0213-8743 (Print); 2605-5686 (Online)
https://www.eweb.unex.es/eweb/extracta/
87. Archives of Control Sciences, ISSN: 2300-2611 (Online)
http://www.degruyter.com/view/j/acsc
88. Asia Pacific Journal of Mathematics, ISSN: 2357-2205 (Online)
89. International Electronic Journal of Algebra, ISSN: 1306-6048 (Online)
90. Jurnal Matematika, ISSN: 1693-1394 (Print); 2655-0016 (Online)
http://ojs.unud.ac.id/index.php/jmat
91. Journal of Algebraic Systems, ISSN: 2345-5128 (Print); 2345-511X (Online)
92. Algebraic Geometry, ISSN: 2313-1691 (Print); 2214-2584 (Online)
93. New Trends in Mathematical Sciences, ISSN: 2147-5520 (Online)
http://www.ntmsci.com/ntmsci/Index
94. Annales Universitatis Paedagogicae Cracoviensis: Studia Mathematica, ISSN: 2081-545X (Print); 2300-133X (Online)
95. Concrete Operators, ISSN: 2299-3282 (Online)
http://www.degruyter.com/view/j/conop
96. Advances in Group Theory and Applications -AGTA, ISSN: 2499-1287 (Online)
http://www.advgrouptheory.com/journal
97. Electronic Journal of Combinatorics, ISSN: 1077-8926 (Online)
The Electronic Journal of Combinatorics
98. Advances in the Theory of Nonlinear Analysis and its Applications - ATNAA, ISSN: 2587-2648 (Online)
99. Discrete Mathematics & Theoretical Computer Science, ISSN: 1462-7264 (Print); 1365-8050 (Online)
http://www.dmtcs.org/dmtcs-ojs/index.php/dmtcs
100. Balkan Journal of Geometry and Its Applications, ISSN: 1224-2780 (Print); 1843-2875 (Online)
101. Nonautonomous Dynamical Systems, ISSN: 2353-0626 (Online)
http://www.degruyter.com/view/j/msds
102. Revista de la Unión Matemática Argentina, ISSN: 0041-6932 (Print); 1669-9637 (Online)
http://inmabb.criba.edu.ar/revuma/
103. Discussiones Mathematicae Graph Theory, ISSN: 2083-5892 (Online)
http://www.degruyter.com/view/j/dmgt
104. Note di Matematica, ISSN: 1123-2536 (Print); 1590-0932 (Online)
http://siba-ese.unisalento.it/index.php/notemat
105. PNA: Revista de Investigación en Didáctica de la Matemática, ISSN: 1887-3987 (Online)
106. Ural Mathematical Journal - UMJ, ISSN: 2414-3952 (Online)
107. Revista Latinoamericana de Etnomatemática, ISSN: 2011-5474 (Print)
http://www.revista.etnomatematica.org
108. Electronic Journal of Differential Equations - EJDE, ISSN: 1072-6691 (Print)
109. Iranian Journal of Optimization - IJO, ISSN: 2008-5427 (Online)
110. Electronic Journal of Graph Theory and Applications - EJGTA, ISSN: 2338-2287 (Print)
111. Annual Review of Chaos Theory, Bifurcations and Dynamical Systems, ISSN: 2253-0371 (Online)
112. Applied General Topology, ISSN: 1576-9402 (Print); 1989-4147 (Online)
http://polipapers.upv.es/index.php/AGT
113. Khayyam Journal of Mathematics, ISSN: 2423-4788 (Online)
114. Electronic Journal of Qualitative Theory of Differential Equations, ISSN: 1417-3875 (Online)
http://www.math.u-szeged.hu/ejqtde/
115. Mathematica Moravica, ISSN: 1450-5932 (Print); 2560-5542 (Online)
http://scindeks.ceon.rs/journalDetails.aspx?issn=1450-5932&lang=en
116. Molecular Based Mathematical Biology, ISSN: 2299-3266 (Online)
https://www.degruyter.com/view/j/cmb
117. Journal of the Belarusian State University: Mathematics and Informatics, ISSN: 2520-6508 (Print); 2617-3956 (Online)
https://journals.bsu.by/index.php/mathematics
118. Journal of Research and Advances in Mathematics Education, ISSN: 2503-3697 (Print)
http://journals.ums.ac.id/index.php/jramathedu
119. Lietuvos Matematikos Rinkinys, ISSN: 0132-2818 (Print); 2335-898X (Online)
https://www.journals.vu.lt/LMR/
120. Osječki Matematički List - OML, ISSN: 1845-4607 (Print); 1848-946X (Online)
http://www.mathos.unios.hr/index.php/en/science/journals/osjecki-matematicki-list
121. UJM (Unnes Journal of Mathematics), ISSN: 2252-6943 (Print)
http://journal.unnes.ac.id/sju/index.php/ujm
122. Discrete Mathematics Letters, ISSN: 2664-2557 (Online)
123. Control and Optimization in Applied Mathematics, ISSN: 2383-3130 (Print); 2538-5615 (Online)
http://mathco.journals.pnu.ac.ir
124. Journal of Interpolation and Approximation in Scientific Computing, ISSN: 2194-3907 (Print)
125. GeoGebra - GGIJRO, ISSN: 2068-3227 (Print); 2247-7241 (Online)
126. An International Journal of Optimization and Control: Theories & Applications - IJOCTA, ISSN: 2146-0957 (Print); 2146-5703 (Online)
http://ijocta.balikesir.edu.tr
127. Journal of Samara State Technical University: Series Physical and Mathematical Sciences, ISSN: 1991-8615 (Print); 2310-7081 (Online)
http://www.mathnet.ru/eng/vsgtu
128. Acta Universitatis Sapientiae: Mathematica, ISSN: 2066-7752 (Online)
http://www.degruyter.com/view/j/ausm
129. Numeracy - Electronic Journal of the National Numeracy Network, ISSN: 1936-4660 (Online)
http://scholarcommons.usf.edu/numeracy/
References:
To be updated
To be updated