The 2021 sixth-round pick will make his first career NFL start in Week 8 against the visiting Washington Commanders.
Veteran starter Matt Ryan, acquired in a trade with the Atlanta Falcons in the offseason, heads to the bench after suffering a shoulder sprain in Sunday's setback at Tennessee.
Regardless of Ryan's injury status, coach Frank Reich said the time is right to make a change for the Colts (3-3-1).
"Right now the move is for Sam to be the starter the rest of the season," Reich told reporters Monday.
Ehlinger, 24, has not played a snap this season and has never attempted a pass in a regular season game.