Nate McMillan will be the interim head coach, taking over a team that is 6-9.
The dismissal came less than a week after Westphal got into a shouting match on the court with star guard Gary Payton in a game in Dallas. The team initially suspended Payton but lifted the suspension before it took effect.
The SuperSonics did not immediately say why they fired Westphal, who had been criticized this season by several of his players.
Westphal, who was in his third year with the team, is the first NBA coach to be fired this season.
Westphal offered to resign Nov. 6 just four games into the season. Sonics president and general manager Wally Walker said Westphal was frustrated by his team's bickering and insubordination but players had talked Westphal out of quitting.
The addition of Ewing was supposed to make the Sonics one of the top teams in the Western Conference. Instead, Seattle has been far from consistent through 15 games.
Ewing is Seattle's first legitimate center since Jack Sikma left after the 1986 season. In his 16th season, Ewing is averaging 31.3 minutes, 10.3 points and 8.9 rebounds. He averaged 15.0 points and 9.7 rebounds for the Knicks last season.
Besides Ewing and Payton, the Sonics also feature Vin Baker as well as young-and-improving Rashard Lewis and top draft choice Desmond Mason. Ruben Patterson and Brent Barry are key players, too.
In 1993, Westphal coached Phoenix to the NBA Finals. Three years later, after 33 games that season, he was fired by the Suns.
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