The team and Gentry "have mutually agreed to part ways," a Suns statement said.
The NBA team is 13-28 this season.
An interim coach was to be named soon, the team said.
The Suns don't play again until Wednesday.
The 58-year-old Gentry, who became interim and then head coach in 2009, has a 158-144 record with the Suns.
Gentry was in the final year of his contract with the Phoenix club.
He previously coached Detroit, Miami and the Los Angeles Clippers.
Millions of Getty images now available for free via embed tool
NBC reportedly holds celebs hostage to Jimmy Fallon's show