A left-footed center back, Callens has earned 10 caps for Peru and made the nation's preliminary roster for the 2016 Copa America Centenario.
"This is a great joy," Callens said in a news release. "It's a new experience that I'm sure will help me maintain a good competitive rhythm and will help me develop more at my position. Of course it's exciting to have a coach like Patrick Vieira with experience at a world level and who will guide me to help improve in my performance.
"I'm a player who pulls out the claws and leaves everything on the field, ready to defend the team, and I have a lot to learn, but I'm sure I'll meet the fans' expectations."
Callens played for Real Sociedad B from 2011 to 2015 before joining Numancia of the Segunda Division, making 40 appearances over the past 18 months.
The 6-foot-1 defender will compete with Maxime Chanot, Frederic Brillant and Ethan White for minutes at center back for NYCFC after the club parted ways with Jason Hernandez and loaned out Jefferson Mena earlier this offseason.