Benrahma enjoyed a standout season in the Championship last term with 17 goals and 10 assists.
Dawson recently fell down the pecking order at Watford under new boss Vladimir Ivic, and the Hornets have now also completed the acquisitions of defenders Francisco Sierralta and William Troost-Ekong.
"I know he will be a great addition to the dressing room - he is a committed and determined character". "I'm really excited and looking ahead to the game at the weekend".
Manager David Moyes has been working this week to try and strengthen his attacking options after a fairly positive start to their Premier League season. "We've got a lot of experienced lads here anyway and to be joining this group of players is an exciting prospect, so I can't wait", he added.
"It's a talented group and I can add competition to that backline, so hopefully I can slot straight in with the lads and help them improve their game as well".
There is already a club-to-club agreement in place between West Ham and Brentford worth around £30million.
Aston Villa has also shown interest in the 25-year-old but the Midlands Club has not found an approach.
King now plays at Championship team Bournemouth and he is a striker who has lots of Premier League experience.
Dawson, who spent the majority of his career at West Bromwich Albion before moving to Watford in 2019, has made over 180 appearances in the top flight. Click the link in the orange box above for details.