The striker has signed a three-year contract to join former Hammers team-mate Nigel Reo-Coker in the West Midlands.
Harewood, 27, has been linked with several clubs since the end of last season and had been expected to join Wigan until Villa closed in on his signature.
Villa have not disclosed the fee they have paid the Hammers for Harewood - who had also been linked with local rivals Birmingham recently.
Villa boss Martin O'Neill told the club's website: "I'm delighted that he's accepted the invitation to come and join us.
"I know there were other teams wanting to take him, but he's prepared to come here and challenge to try and work his way into the first team.
"I'm delighted to have him. He's a strong lad, very athletic and if he can recapture the form he showed in the two seasons before last, he will definitely be a big asset."
O'Neill added: "He's coming to join a squad. We've let some players go during the summer and we're now rebuilding the squad again.
"Marlon's prepared to come in here and battle for his place and that's what we want at this football club. He's a confident lad who's willing to fight for his place."
Harewood agreed with O'Neill's assessment that he must strive to recapture the form of two seasons ago.
"I'm delighted to join Villa and I think I've come at a really exciting time for the club," he told the club's website.
"The manager is aiming high next season and it's great to be part of his plans.
"I want to recapture the form I showed a couple of seasons ago, when West Ham got into the cup final and we did so well in the league.
"There's a lot of quality here at Villa. I watched the lads train this morning and I'd forgotten just how much quality there was already here.
"My mate Nigel Reo-Coker has already signed and I know all about his qualities, so I'm just really pleased to be on board.
"Villa finished last season strongly and I know the aim is to get into Europe this time around. Hopefully I can play my part in that."
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