The fullback has penned a seven-year contract and joins for a fee of £15m.
“It was a no brainer for me to join West Ham – I’m excited and happy to be here,” said the 26-year-old, who will wear the No29 shirt.
“It’s an amazing feeling to be back in London, and I’m excited for what’s coming. I was born here, so I know the ins and outs of London, which plays a big part in my life, so to be back playing in this city means a lot to me.
“I’m happy to be here, am grateful for the warm welcome, and I am excited to be playing for the Club and will give it my all.
“I can’t wait to get on the pitch, get to know the players, and push on from there. I see a team that is in it together, that will push each other to win, and a group that has got each others' backs through thick and thin, so being part of that and having that insurance helps a lot when you are surrounded by a good squad.”
Hammers Technical Director Tim Steidten also told the club's website: “I am delighted we’ve been able to get the deal to sign Aaron over the line.
“He’s exactly the type of player we’re looking to attract to this Club in the prime years of his career.
“He’s an incredibly talented defender who we rate very highly – superb in one-on-one situations. He’s versatile too, which is clearly another positive. He’s played over 170 Premier League games during his career – he knows the division inside-out.
“I’d like to personally thank the Board for their continued backing to bring players of such quality to West Ham. It’s a really exciting time for us as we prepare for the start of the new season this weekend.”