The Italian leaves Leicester City after guiding the Foxes to Premier League promotion as Championship title winners.
Maresca has penned a five-year Chelsea contract with the option of an extra year.
He replaces Mauricio Pochettino, who left by mutual consent on 21 May after just one season at Stamford Bridge.
Maresca said: "To join Chelsea, one of the biggest clubs in the world, is a dream for any coach. It is why I am so excited by this opportunity.
"I look forward to working with a very talented group of players and staff to develop a team that continues the club's tradition of success and makes our fans proud."
Laurence Stewart and Paul Winstanley, Chelsea's co-sporting directors, said: "We are delighted to welcome Enzo to Chelsea. He has proven himself to be an excellent coach capable of delivering impressive results with an exciting and identifiable style.
"Enzo has deeply impressed us in our discussions leading up to his appointment. His ambitions and work ethic align with those of the club. We thoroughly look forward to working with him."
Chelsea's ownership group also stated: "We are thrilled to welcome Enzo into the Chelsea family. We look forward to supporting him and the rest of the sporting team in fulfilling their potential and our expectations over the years to come.
"He is a highly gifted coach and leader that we are confident can help fulfill our vision and competitive goals for the Club."