Diame moves to the KC Stadium on a three-year deal following a two-year spell with West Ham in which he made 79 appearances for the east London club.
The Senegal international now becomes Hull's second signing of the final day of the English transfer window following the club-record capture of striker Abel Hernandez earlier on Monday.
And manager Steve Bruce is confident that the former Wigan Athletic midfielder will be an asset to the Yorkshire club as they bid to maintain their Premier League status after finishing just four points above the drop zone in their first season back in the top flight last term.
"I'm delighted to welcome Mohamed to the Club," Bruce said.
"His experience, both at international level and in the Premier League will be invaluable for us as we look to cement our place in the top-flight."
Diame could make his Hull debut against old club West Ham when Sam Allardyce's men travel to Humberside in two weeks' time.
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