He achieved almost 30,000 more votes than independent candidate George Ferguson who has held the leadership position since 2012.
Both first and second preference votes were counted across two rounds, with the pair left to a two-horse race in a copy of what happened four years ago.
Mr Rees received a total of 68,750 votes and Mr Ferguson 39,577 from 44.87% of the electorate who voted.
In his victory speech, Mr Rees said it had been "an incredible journey" which started many years before his first bid to take leadership of Bristol City Council in 2012.
He paid tribute to all his supporters and said the "collective action" had been humbling.
Mr Ferguson followed with a speech in which he quipped: "I want to thank you for giving me my life back."
He said to Mr Rees: "I hope you don't have to put up with some of the barracking and bile that I've had to put up with. I'll be the first there to defend you if you do."
Labour's Marvin Rees has been elected as Bristol city's mayor.