Fabianski, who made 216 appearances in a seven-year spell with West Ham, was one of many senior players who were released by the club when their contracts expired at the end of the 2024-25 campaign.
However, West Ham decided to offer a one-year deal when they had only two senior goalkeepers in the squad - Mads Hermansen and Alphonse Areola - after Wes Foderingham joined Cyprus side Aris Limassol on a permanent deal on Tuesday.
"Regardless of him being here previously, the main factor is that he is still a top goalkeeper and a top professional, and the perfect choice for the role we needed to fill," West Ham coach Graham Potter said in a statement.
"It's a signing that makes perfect sense for everyone. Lukasz knows the club extremely well, is very well respected and popular among everyone here. His personality and character will be a great example, especially to our younger players.
"He is someone who really cares about West Ham United and wants to help us in any way he can. We are very happy to have him back with us."
West Ham have conceded a league-high eight goals this season with new recruit Hermansen in goal. They sit 16th in the standings ahead of Saturday's London derby against Tottenham Hotspur.