Ciani, 33, has spent time in France, Spain and Italy. He will occupy an international roster spot and will begin training with the club this week.
Ciani made more than 100 appearances in France with FC Lorient and FC Bordeaux. He played with Lazio in Serie A, RSC Charleroi in Belgium and Espanyol in Spain.
“We targeted and acquired Michael as a player who can immediately strengthen our backline,” Galaxy General Manager Pete Vagenas said. “He is a valuable signing that will be an important piece of our roster. We are happy to add him to our team and look forward to him joining the squad next week.”