The network said it has ordered 12 one-hour episodes of each new program. Both are to premiere next year.
The announcement was made Monday by David Nevins, Showtime's president of entertainment.
"'Ray Donovan' and 'Masters of Sex,' each in their own distinctive ways, represent the caliber of programming we want to bring to our subscribers," Nevins said in a statement. "Both series are ambitious and expansive, and feature great writing and great acting. I'm so excited for viewers to experience these two new dramas, which I believe will prove to be fantastic complements to our current slate of programming."
In "Ray Donovan," Schreiber plays a professional "trouble shooter" called in to solve the complicated, confidential and controversial problems of Los Angeles' elite. The series co-stars Jon Voight, Elliot Gould, Eddie Marsan, Dash Mihok and Paula Malcomson.
"Masters of Sex" stars Michael Sheen and Lizzy Caplan as William Masters and Virginia Johnson, real-life pioneers of research in the study of human sexuality.
Rosie O'Donnell unveils nearly 50-pound weight loss
Members of Congress to keep receiving porn magazine