Flamini had been without a club since being released by Crystal Palace at the end of last season, but the Frenchman will now look to help his new side finish in the top half of the La Liga standings.
At a press conference, Getafe boss Jose Bordalas told reporters: "He has arrived, he is with us. He will start with the group tomorrow.
"He comes to help us, to contribute, and we are happy. His experience can help us."
The transfer means that Flamini will play football outside of the Premier League for the first time since 2013.
Getafe are next in action when they face Leganes on Sunday.