It is understood the Hammers have agreed to pay £25m upfront, with another £5m in add-ons.
He will have a medical before the end of Thursday.
There had been talking of a dispute between Said Benrahma, who scored 17 goals in 43 Championship appearances last season, and Brentford.
Brentford will make a massive profit on the 25-year-old, who cost them less than £3m when he arrived from Nice two years ago.
He was a central figure in the Bees’ charge towards promotion last year, which delay at the end of the season when they lost their final two games when two points would have been enough to take them into the top flight for the first time in their history.
Brentford later lost to Fulham in the play-off final. They are currently 14th in the table with four points from their opening four games.
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