With regular number one Rob Green sidelined for up to six weeks with a knee injury, manager Sam Allardyce has moved quickly to bring in Almunia on an emergency deal.
The 34-year-old, who has been at Arsenal since 2004, will be in the squad for the Hammers' trip to Crystal Palace on Saturday.
Allardyce was pleased with his 13th signing, and also sent his thanks to the Gunners for allowing the move.
"We've been hot on the phone and secured an excellent signing in the short term," Allardyce told West Ham TV.
"It's a big thank you to Arsene Wenger and Arsenal for allowing us to get such a high-class and experienced goalkeeper to come and fill in for Robert for the time being."