NFL.com reported that the deal is for one year.
The 6-foot-2, 210-pound Cromartie is on the back end of his career after playing last season for the New York Jets, who released him Feb. 21.
The 32-year-old has seen action in 158 career NFL games and made 133 starts, recording 389 tackles, 31 interceptions (including three returned for touchdowns), 115 passes defensed, two forced fumbles and five fumble recoveries (one returned for a touchdown) for the Jets (2010-2013, 2015), Arizona Cardinals (2014) and San Diego Chargers (2006-09).
The four-time Pro Bowl selection (2007, 2012, 2013, 2014) was named first-team All-Pro in 2007 by the Associated Press and Sports Illustrated. His 31 interceptions rank third in the NFL among active players.
The Colts' secondary has been riddled by injuries in the preseason and there is concern about the front seven's pass-rushing ability.
The aging Cromartie is reported to be slowed by hip problems. He struggled last season with the Jets and had drawn little interest in the free agent market this spring and summer before working out with the Colts on Monday.
Indianapolis starting cornerbacks Vontae Davis (sprained knee) and Patrick Robinson (groin) are nursing injuries. Reserves Darius Butler (hamstring) and D'Joun Smith (knee) also are slowed in preseason by injuries, forcing the Colts to turn to undrafted rookie Darius White last Saturday during a 19-18 preseason loss to the Baltimore Ravens.
"It's not pretty right now," Indianapolis coach Chuck Pagano said.
The Colts made several other transactions Monday, signing undrafted free agent wide receiver Justin Berger and waiving wide receivers Marcus Leak and Andrew Opoku.
Berger played for Diablo Valley College in Pleasant Hill, Calif., before appearing in 27 games for Wyoming. He managed seven receptions for 90 yards and a touchdown with the Cowboys.
Both Leak and Opoku signed as free agents earlier this year. Leak didn't see any preseason action, while Opoku assisted on a special teams tackle Saturday.