Saying it has been "by far the most challenging few weeks in my professional life," Durant announced his decision Monday in a post on The Players' Tribune.
He is expected to sign a two-year, $54.3 million contract, sources told ESPN's Marc Stein. The deal would include a player option after the first year.
"The primary mandate I had for myself in making this decision was to have it based on the potential for my growth as a player -- as that has always steered me in the right direction," Durant wrote. "But I am also at a point in my life where it is of equal importance to find an opportunity that encourages my evolution as a man: moving out of my comfort zone to a new city and community which offers the greatest potential for my contribution and personal growth.
"With this in mind, I have decided that I am going to join the Golden State Warriors."
After Durant signs with Golden State, two-time reigning MVP Stephen Curry will become the fourth-highest-paid player on the Warriors. Durant will be No. 1, followed by Klay Thompson ($16.6 million), Draymond Green ($15.3 million) and Curry ($12.1 million), who can become a free agent after next season.
Durant's contract can be finalized July 7, when a leaguewide moratorium is lifted on signings and trades.