The moves were announced Tuesday, Indy’s regular day off.
The Colts (2-3) also waived outside linebacker Jerry Brown and released tight end Dominique Jones from the practice squad.
The 6-foot-5, 270-pound Saunders played in 16 games and made six starts last season with Pittsburgh. He had four receptions for 29 yards and one touchdown.
Harnish, who grew up in Bluffton, Ind., was the final draft selection in April and made the active roster but was inactive for all five games this season.
Brown, an undrafted free agent, made his NFL career debut Sunday.
Jones made one start, caught one pass for eight yards and played in four games this season for Indy.
Source: Associated Press