This list is by no means definitive — there's a ton more people who have had a role in both a Marvel and a DC movie!
Let's start off with a pretty easy one.
1. Ryan Reynolds

2. Chris Evans

3. Alice Braga

4. Angela Bassett

5. Ben Affleck

6. Ben Mendelsohn

7. Brian Tyree Henry

8. Brett Cullen

9. Chris Pine

10. Djimon Hounsou

11. Christian Bale

12. Florence Kasumba

13. Danny Huston

14. Halle Berry

15. David Dastmalchian

16. David Harbour

17. James Marsden

18. Jared Leto

19. Idris Elba

20. J.K. Simmons

21. Laurence Fishburne

22. Joe Manganiello

23. Jon Favreau

24. Josh Brolin

25. Michael Fassbender

26. Pom Klementieff

27. Michelle Pfeiffer

28. Michael Keaton

29. Patton Oswalt

30. Michael Rooker

31. Rachel Weisz

32. Randall Park

33. Taika Waititi

34. Russell Crowe

35. Sean Gunn

36. Tilda Swinton

37. Sylvester Stallone

38. Tom Hardy

39. Zoë Kravitz

40. Tommy Lee Jones

41. Willem Dafoe

42. Zachary Levi

43. Zazie Beetz

44. Zoe Saldaña

45. Stan Lee

46. Coolio

47. Corey Johnson

48. Kenneth Choi

49. Tao Okamoto

50. Terence Stamp

51. Mark Hamill

52. Natalie Portman

53. Hugo Weaving

As I mentioned, this isn't even all of the actors with both Marvel and DC roles! Are you surprised there's so much crossover?
Who were you most surprised to see on this list? Let me know in the comments!