The Argentinean spent the first half of the campaign on loan at AC Milan, where he scored eight goals.
Upon completing his move, Higuain, who will wear the number nine shirt, said: "When the opportunity to join Chelsea presented itself I had to take it. It’s a team I’ve always liked that has a lot of history, a wonderful stadium and they play in the Premier League, a league I’ve always wanted to play in.
"I now hope I can give back that trust Chelsea have shown me out on the pitch. I can’t wait to start and I hope to adapt as soon as possible."
Director Marina Granovskaia said: "Gonzalo was our number one target in this transfer window and arrives with a proven record at the highest level. He has previously worked with Maurizio to great effect and is familiar with how the coach likes to play.
"This was not an easy deal for us to do because of the numerous parties involved, but we are delighted we were able to make it happen and we look forward to seeing the impact Gonzalo will make for us in the second half of the season."