The Baggies have gathered seven points from 13 league games, with eight defeats, though they earned a surprise 1-1 draw at Manchester City on Tuesday.
Assistant coaches Dean Racunica and Danilo Butorovic also depart, as does first-team coach Julian Dicks.
A club statement thanked Bilic and his coaching staff "for their efforts in achieving promotion last season".
Bilic, 52, signed a two-year contract when he took over at the The Hawthorns in June 2019, replacing Darren Moore.
He guided Albion to promotion on the final day of last season, as they finished second in the Championship.
But they have won only one Premier League match this season, beating bottom club Sheffield United 1-0 on 28 November.
It emerged after the draw at Manchester City that West Brom were considering whether to sack Bilic.
Tuesday's match was the Croat's 100th Premier League game as a manager. He led West Ham from June 2015 to November 2017, taking the club into Europe as they finished seventh in his final full season, recording their highest placing since 2002.
Prior to that, he had led Croatia to two European Championships, with a quarter-final appearance in 2008 coming after they had knocked England out in the qualifiers.
He has also had spells in charge of Hajduk Split, Lokomotiv Moscow, Besiktas and Al-Ittihad.
Sam Allardyce, Nigel Pearson, Mark Hughes and Eddie Howe are all reportedly
candidates to replace Bilic.
Allardyce has been out of work since leaving Everton in the summer of 2018, while Hughes left Southampton later the same year.
Pearson, who is a former Albion caretaker-manager, had a 20-game spell at Watford last term, while Howe left Bournemouth in August after his side were relegated from the Premier League.
Manchester United manager Ole Gunnar Solskjaer said he was "very sad" to hear of Bilic's sacking.
"I think he's done great for them, getting promotion," Solskjaer said. "I'm sure he'll get a job again when he wants to.
"Hopefully more and more clubs and teams think long term. Of course it's a short-term business, we need results short-term as well, but everyone knows that continuity is key to success.
"Sometimes they don't have the patience for it, though."
| Bilic's managerial record at club level (all competitions) | |||||
|---|---|---|---|---|---|
| Club | Played | Won | Drawn | Lost | Win ratio |
| Hajduk Split (2001-02) | 17 | 11 | 4 | 2 | 65% |
| Lokomotiv Moscow (2012-13) | 32 | 13 | 7 | 12 | 41% |
| Besiktas (2013-2015) | 92 | 48 | 22 | 22 | 52% |
| West Ham (2015-17) | 111 | 42 | 30 | 39 | 38% |
| Al-Ittihad (2018-19) | 17 | 3 | 5 | 9 | 18% |
| West Brom (2019-20) | 65 | 26 | 22 | 17 | 40% |
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