Keegan, who signed Beardsley when he was Newcastle's manager and played with him at St James' Park before that, has persuaded the 37-year-old former England striker to join the London club from Bolton until the end of the season.
"It's great news for us," said Keegan. "I'm joining up again with a player I've spent many, many games with. We've taken him until the end of May because the play-offs come pretty late and I said to Peter: 'Do you fancy going back to Wembley, because that's where we're going'. It didn't take him long to decide, and he's absolutely delighted as well. He's still as fit as a fiddle and a tremendous inspiration to those around him. His enthusiasm and the way he works with younger players will be a real boost for us."
Keegan, Fulham's director of football, has tried to sign the former Liverpool and Everton striker previously this season, but lost out on more than one occasion. "It's a massive coup for this club and shows that we will try to get anybody for Fulham," Keegan added.