The 32-year-old, who spent two years with the Hammers before leaving to join Boro in 2017, returns for an undisclosed fee and has agreed a three-and-a-half-year contract.
“I’m delighted to be back at West Ham United. I had no hesitation in coming back. West Ham is a massive club, with fantastic supporters and an excellent squad of players,” Randolph told the club website.
“This opportunity was simply too good to turn down, though. It’s a great feeling to be back in the Premier League with such a fantastic club, and I can’t wait to get started.”
The transfer was completed on Wednesday following reports last week that the move was put on hold after issues arose during Randolph’s medical.
West Ham wanted to be sure he would not break down once he joined, having recently had a thigh injury. But Randolph is fit again and, with Lukasz Fabianski out until next month with a hip injury, he could go straight into the team ahead of David Martin when the Hammers host Everton in the Premier League on Saturday.
“It’s a great challenge for me and I am really looking forward to working with the goalkeepers here and helping the team in any way I can,” said the Republic of Ireland international.
“I’d like to thank everyone at Middlesbrough - the management, staff, players and fans - for their support. I had a great time there and wish them all the very best for the rest of the season.”