The 37-year-old American returns to the Premier League after spells with Tottenham and Leicester City.
Keller added: "It's fantastic to be back in the Premiership after a couple of year's absence."
Fulham have been short of keepers following an injury to Antti Niemi and Keller will be able to provide vital cover for Tony Warner.
"With Antti being injured, it was important we added some strength and depth," said Sanchez.
"Kasey is a proven professional with impeccable international and he will be a valuable asset to us over the course of the season."
The American said of his return to London: "I am really enthusiastic about this new challenge."