Hunter, who played for 17 seasons in the NBA, joined the Suns in the scouting department last year and this season took over the team's new player development department.
As a guard with five NBA teams, Hunter averaged 8.5 points in 937 games, 438 as a starter. He won NBA championships with the Los Angeles Lakers in 2002 and the Detroit Pistons in 2004. Hunter, 40, has no prior coaching experience.
He retired in 2010 and served in a player development job with Detroit before coming to Phoenix.
The team announced Hunter's promotion Sunday before its first practice since a home loss to Milwaukee on Thursday night, the Bucks' first win in Phoenix in 25 tries.
The Suns, with nine new players on their roster this season, have lost 13 of 15, including four straight at home, and at 13-28 have the worst record in the Western Conference. They don't play again until Wednesday, when they face the Kings in Sacramento. Hunter's home debut will come Thursday night against the Los Angeles Clippers, who have the West's second-best record at 32-9.
Gentry was Phoenix's coach since Terry Porter was fired at the 2009 All-Star break. His time included a run to the 2010 Western Conference finals in 2010, his first full season as coach. But the team failed to make the playoffs the last two seasons. Then Steve Nash departed to the Los Angeles Lakers, and the Suns, under the direction of team president Lon Babby and general manager Lance Blanks, overhauled the roster.
Meanwhile, owner Robert Sarver has had to face dwindling crowds at US Airways Center.
Gentry, good natured and popular despite the team's lack of success, tried a series of lineups to develop some sort of chemistry and consistency with no success, and he acknowledged that he understood it was time for a change.
Gentry tweeted Sunday morning that he was ''already bored.''
''Need some suggestions as to what to do to fill the day,'' he wrote. ''Let's hear some good ones.''