Saying he has experienced "by far the most challenging few weeks in my professional life," Durant announced his decision to leave the Oklahoma City Thunder on Monday in a post on The Players' Tribune.
Durant is expected to sign a two-year, $54.3 million contract, sources told ESPN's Marc Stein. The deal will 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 be 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.
Green told The Undefeated's Marc J. Spears in a text message that he was ecstatic the Warriors landed Durant. He tweeted to express his enthusiasm about his new teammate.
"I'm excited about the opportunity to add one of the best players in the world to our team and welcome him to our brotherhood! This will be some of the best times of our life, and I'm looking forward to it," he wrote.
Durant's contract can be finalized Thursday, when the leaguewide moratorium on signings and trades is lifted.
Warriors coach Steve Kerr was in Hawaii and said he learned about Durant's decision to join Golden State via The Players' Tribune, ESPN's Ramona Shelburne reported.
The Warriors plan to renounce their rights to Harrison Barnes, sources told Stein, to help clear the salary-cap space needed to sign Durant. That will make Barnes an unrestricted free agent on Thursday. Barnes, sources told Stein, remains on course to sign with the Dallas Mavericks, who reached a verbal agreement with the 24-year-old on a four-year max deal worth $94.4 million, no matter what happened with Durant.
The Mavericks also have agreed to a trade with the Warriors to acquire center Andrew Bogut and his 2016-17 contract, with a base value of just over $11 million. Just before Durant's Players' Tribune post, Bogut removed references to the Warriors from his Twitter page.
Golden State will renounce its rights to center Festus Ezeli, which will make him an unrestricted free agent, but guard Shaun Livingston was told by the Warriors that he won't be traded, sources told Spears. Golden State also agreed to a one-year, $2.9 million deal with Zaza Pachulia, according to a report by The Vertical.
Thunder general manager Sam Presti expressed his team's disappointment over Durant's departure in a statement.
"Kevin made an indelible mark on the Thunder organization and the state of Oklahoma as a founding father of this franchise," Presti said. "We can't adequately articulate what he meant to the foundation of this franchise and our success. While clearly disappointing that he has chosen to move on, the core values that he helped establish only lead to us thanking him for the many tangible and intangible ways that he helped our program."
Presti later said the team had a pretty good idea that Durant was moving on even before its final meeting with him in the Hamptons on Sunday.
"Even when we were relatively sure it probably wasn't going to go our way, eight staff members and our owner stayed overnight in New York just to be there and to kind of finish it out," Presti said at a news conference in Oklahoma City on Monday evening.
"We've been there for every first in his career, and we wanted to finish it out the right way by being present whenever that decision was made, regardless of how it turned out."
Presti said he was alerted to Durant's final choice in a phone call from Durant on Monday morning a few minutes before the announcement.
"It was disappointing, it was hurtful; but at the end of the day, it was what he felt was best for him," Presti said.
The biggest catch on the free-agent market, Durant met with six teams over the past three days: Oklahoma City, Golden State, San Antonio, Boston, Miami and the Los Angeles Clippers. The Warriors held their meeting with Durant on Friday.
On Saturday, Durant spoke with NBA legend Jerry West, a member of the Warriors' executive board, on the phone, sources told ESPN's Chris Broussard. West outlined the reasons he thought Golden State would be the ideal fit for the superstar forward.
Durant, 27, has spent all nine seasons of his career with the Thunder organization. He won the MVP in 2014, has been named first-team All-NBA five times and has made seven All-Star teams. He has appeared in four Western Conference finals and one NBA Finals, in which he lost in five games to the Heat in 2012.
"I'm from Washington D.C. originally, but Oklahoma City truly raised me," Durant wrote. "It taught me so much about family, as well as what it means to be a man. There are no words to express what the organization and the community mean to me and what they will represent in my life and in my heart forever. The memories and friendships are something that go far beyond the game. Those invaluable relationships are what made this deliberation so challenging.
"It really pains me to know that I will disappoint so many people with this choice, but I believe I am doing what I feel is the right thing at this point in my life and my playing career."
Durant bounced back last season to earn second-team All-NBA honors after undergoing three surgeries to repair a Jones fracture on his right foot suffered before the 2014-15 season, in which he was limited to 27 games before undergoing a season-ending bone graft procedure in March. Without Durant, the Thunder missed the postseason.
With Durant back for 72 games last season, the Thunder improved to 55-27 and finished third in the West. The Thunder beat the Dallas Mavericks in five games in the opening round, upset the 67-win Spurs in six games in the second round and pushed the Warriors to seven games in the Western Conference finals.
Durant averaged 28.2 points, 8.2 rebounds and 5.0 assists last season, and he scored at least 20 points in 67 straight games. In nine seasons, Durant has averaged 27.4 points, 7.0 rebounds and 3.7 assists per game.
Golden State set an NBA regular-season record with 73 wins and led the league in scoring last season, but the Warriors fell just short of a repeat NBA title and lost to the Cleveland Cavaliers in seven games.
ESPN staff writer Royce Young and The Associated Press contributed to this report.