The 34-year old will be joining the Impact in return for allocation money, ending a three-year spell with the Galaxy who took out this season's MLS Cup with Ricketts sitting on the bench as Bruce Arena preferred Josh Saunders in goal.
"Donovan is one of the best goalkeepers in the league. We are very pleased to have made this acquisition," said Impact head coach Jesse Marsch. "He will be an important piece to our team in 2012. We value him greatly and know that he will give us experience and quality in front of the net.
"We are happy to confirm today the arrival of another important player in a key position," said sporting director Nick De Santis. "From day one, we spoke about having influential players on this team. We feel that it is important to have a first-class goalkeeper with MLS experience. Donovan has played for one of top clubs in MLS. With this acquisition, we are taking another positive step forward in building the foundation of our team."