The trade occurred after reaching a new four-year, $28 million contract, sources confirmed to ESPN.com's Brian Windhorst.
In return, the Kings acquire a $7 million trade exception and the rights to forward Alex Oriakhi, the 57th overall pick in 2013.
Thomas averaged 20.3 points and 6.3 assists per game last season, both career-highs.
Thomas took to Twitter on Friday to thank the Kings and express his excitement in a series of tweets:
God is great!!! I'm excited for what the future holds here in Phoenix. Proud 2 be apart of the @Suns organization! #Playoffs#Suns#TMC
- Isaiah Thomas (@Isaiah_Thomas2) July 11, 2014Thomas also thanked the city of Sacramento, saying the city "will always have a place in my heart."
"To the fans of Sacramento, words can't explain what y'all mean to me!," he tweeted. "Y'all made me who I am in the NBA and I can't thank you guys enough."
He thanked Sacramento Mayor Kevin Johnson, a former star point guard for the Suns, for "being by my side since Day 1."
The move leaves six point guards on the Phoenix roster, including the double point guard starting lineup of Goran Dragic and Eric Bledsoe.
Ish Smith, the third point guard last season, could well be gone. Phoenix has two young point guards in this year's first-round draft pick Tyler Ennis and second-year pro Archie Goodwin.
Point guard play is critical to the high-energy, fast-paced system of coach Jeff Hornacek, who in his first season on the job led Phoenix to a big turnaround. The Suns, coming off a 25-win season in 2012-13, went 48-34 and barely missed the playoffs in the tough Western Conference.
Bledsoe is a restricted free agent, but the Suns are trying to sign him before he inks an offer sheet for another team. General manager Ryan McDonough has said Phoenix would match any offer sheet Bledsoe signs.
Bledsoe's agent is Rich Paul, who also represents LeBron James.
On Thursday, the Suns reached agreement on a three-year contract with their starting small forward, P.J. Tucker, who was a restricted free agent.
Information from the Associated Press was used in this report.