The move was first reported by The Arizona Republic on its Web site Sunday.
The Suns were attempting, without success, to avoid making news over the All-Star weekend being staged on their home turf. The franchise also is in talks with numerous teams about trading Amare Stoudemire, a starter in Sunday night's All-Star Game.
The Phoenix power forward, being shopped around the NBA in advance of Thursday's trade deadline, scored 19 points on 7-for-10 shooting in 24 minutes for the West in his team's 146-119 All-Star victory over the East Sunday night.
He is just 26 years old, and he wondered on Friday why he has lost his status as foundation of a franchise.
"If my last home game is as an All-Star starter here, that would be a great way to go out," Stoudemire said.
Porter was four months into his first season on the three-year, $6 million contract he got to replace Mike D'Antoni, who left after four highly successful seasons to coach the New York Knicks.
Gentry, extremely popular with Suns players, has been a coach in the NBA for 20 years, including interim head coaching stints with Miami and Detroit. He was head coach of the Los Angeles Clippers from 2000 to 2003. A team source told Bucher that Gentry will be the head coach through the remainer of the season.
"I think Alvin is a great players' coach," Stoudemire said after the All-Star Game. "He gets along with his players well. Of course, he'll have a great game plan from a basketball standpoint. I think it will be somewhat up-tempo."
Asked if the coaching change was needed, Stoudemire said, "Only time can tell."
"I think Terry Porter definitely put a lot of hard work in here to try to get us on the right track," he said, "and I'm pretty sure Alvin's going to do the same."
Gentry was the only holdover from D'Antoni's staff in Phoenix. His hiring as assistant coach in 2004 coincided with the arrival of Steve Nash and the subsequent ultra up-tempo offense that propelled the Suns to 54 wins or more for each of the past four seasons.
The decision to replace Porter came in a series of meetings between owner Robert Sarver and general manager Steve Kerr in which they reviewed the team's entire basketball operation. The two are considering trading Stoudemire in a move designed to clear salary-cap space for future signings.
Chicago, Cleveland, Miami and Portland are among the teams mentioned as possible destinations for the Suns' power forward.
It marks the second time Porter has been fired as an NBA coach. He was let go by the Milwaukee Bucks in 2005 after two seasons.
Sarver and Kerr could not be reached by ESPN.com on Sunday for comment.
Porter, who played in the NBA for 17 seasons, was an assistant with the Detroit Pistons when he was hired by the Suns, and the intention was for him to bring the Pistons' defense-oriented, slower playing style with him.
That plan was jettisoned after it became apparent the Suns' personnel was not suited to the slowdown game.
The promotion of Gentry means the Suns will be even more committed to a fast-paced game.
Phoenix (28-23) lost five of eight going into the All-Star break. The Suns trail Utah by one game for the eighth and final playoff spot in the West.
Earlier Sunday, Porter told The Associated Press he hadn't heard anything from his bosses.
"I know they're meeting tonight," he said. "They're reviewing the whole system."
Information from ESPN The Magazine's Ric Bucher, ESPN senior NBA writer Marc Stein and The Associated Press was used in this report.