Fabianski boasts plenty of top-flight experience, having spent more than a decade with Swansea and former club Arsenal, and Pellegrini hopes that the 33-year-old can put his age to good use at the London Stadium.
"With Lukasz, we have secured a proven Premier League goalkeeper who has great experience at this level," he told the club's official website.
"We believe he has a number of good years ahead of him - hopefully his best - and there is no doubt that he will give us excellent competition for the number one spot.
"It is important, as a Premier League club, to have strong goalkeepers who can push each other and we have that dynamic with Lukasz and Adrian now."
No Premier League goalkeeper has made more saves than Fabianski's 499 over the past four seasons.