New York entered Saturday night's game against Boston with its first basemen hitting .174 with five homers. The group's .531 OPS ranks 29th in the majors, ahead of only Cleveland.
The 6-foot-4, 250-pound Gittens was set to bat sixth and make his major league debut against the Red Sox.
"He's earned this opportunity,'' Yankees manager Aaron Boone said. "He can continue to earn more.''
Luke Voit led the majors in homers as the Yankees' first baseman in 2020 but has played in just 12 games this season. He had left knee surgery in March and missed the start of the season, and he's currently back on the injured list with a right oblique strain.
Gittens was a 12th-round pick by New York in 2014, and he's shown impressive power despite a slow climb up the minor league ladder. He has hit .265 with a .476 slugging percentage over seven seasons, including .283 with four homers, a .585 slugging percentage and a 1.071 OPS this season with the RailRiders.
He spent last summer at home in Texas, keeping sharp during the pandemic shutdown by hitting off a tee he purchased at a store and set up with a net on the roof a parking garage.
"My biggest thing was be ready,'' he said.
"They get walked a lot, they get thrown junk a lot, and they don't bow down to it,'' Gittens said.
Gittens is wearing No. 92, the second highest number worn by a Yankee behind the No. 99 currently worn by Judge and previously worn by Brian Bruney (2009) and Charlie Keller (1952).
Right-hander Corey Kluber (strained shoulder) was transferred to the 60-day injured list to make room. The move was expected, with Kluber likely out until at least late July.