The people, who sought anonymity because the deal has not been officially announced, say Thomas is signing a four-year, $27 million contract. In exchange, the Kings get the draft rights to center Alex Oriakhi, a second-round draft pick of the Suns in 2013, plus a $7 million trade exception.
One of the people who confirmed the deal said the Suns see Thomas as a "perfect third guard" to play with Phoenix's double point guard starting lineup of Goran Dragic and Eric Bledsoe. He is not envisioned as a replacement for either of them.
Thomas was a restricted free agent who was in Phoenix meeting with the Suns on Thursday and Friday.
The Kings were not expected to match any offer sheet for Thomas after agreeing to a three-year, $15sign free agent Darren Collison last week. With Friday's agreement, the Kings at least get something for Thomas.
Thomas was the 60th and final draft pick, out of Washington, in 2011 and became a fan favorite as he worked his way into more and more playing time in Sacramento.
He averaged career highs of 20.3 points and 6.3 assists in 72 games with Sacramento last season, 54 as a starter. In three seasons with the Kings, he played in 216 games, 153 as a starter, averaging 15.3 points and 4.8 assists.
"I'm excited for what the future holds here in Phoenix," he said in a tweet.
Thomas 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 said. "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. 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.
The Kings are counting on some of their young guards to blossom in Thomas' place.
Last year's second-round pick, point guard Ray McCallum, showed signs of improving with more minutes in the final weeks of the season. And Sacramento has drafted a shooting guard with its last two first-round picks, Ben McLemore (seventh overall) out of Kansas last year and Michigan's Nik Stauskas (eighth overall) this June.
Kings owner Vivek Ranadive, general manager Pete D'Alessandro and coach Michael Malone have made as many moves as any team in the league since taking over the Kings last year, so Sacramento could shuffle its roster with more trades.