The Pirates optioned infielder Brent Morel, who went 2-for-2 with three runs batted in Saturday night in an 8-3 over Arizona, to Class AAA Indianapolis and designated infielder Dean Anna for assignment to add Nix to the 40-man roster.
Morel was 3 -for 16 in nine games with the Pirates.
Nix has played third base, shortstop and second base this season. He signed with Tampa Bay on May 17 and hit .274 in 54 games with AAA Durham.
Nix was expected to be active for Sunday's game against the Diamondbacks.