The minor league deal includes an invite to big league camp, where he will compete for a place on the Reds' bench.
"He provides great depth. He's been a shortstop, he's played second base, third base," Reds manager David Bell said Saturday. "He's been a successful major league player on really good teams. He brings experience of winning. If he makes the team, he will make us a deeper, better team."
Bell made it clear that Jose Peraza will be the Reds' starting shortstop.
"Peraza has made great strides. We believe in what he's doing," Bell said. "We need to be as strong as we can with all 12 guys that make our team as position players."
Iglesias hit .269 with five home runs and 48 RBIs for the Detroit Tigers in 2018. In 656 major league games with the Tigers and Boston Red Sox, he has hit .270 with a .315 on-base percentage. He was an All-Star in 2015.
"I'm looking forward to the opportunity to help this team be special this year," Iglesias said.
The 29-year-old Iglesias, who defected from Cuba to play baseball for the Red Sox, made his big league debut in 2011 with Boston. He was traded to the Tigers in 2013 and finished second in the American League rookie of the year vote.
It has been five years since Iglesias played any other position but shortstop. He played 37 games at third base and three games at second base for Boston in 2013.
Being versatile will add to Iglesias' chances of earning a job with the Reds.
"We will figure it out. At the end of the day it's about helping the team win a game," Iglesias said about changing positions.
The Associated Press contributed to this report.