The 23-year-old former Rovers trainee, who left Ewood Park to join the Eagles in August 2010, has signed a three-year contract with the Championship outfit.
He did not feature for the senior team during his first spell at Blackburn, but he is likely to be given his debut in the near future by boss Gary Bowyer, who managed Marrow at reserve level.
"Alex is someone I know well from back in the youth and reserve team days here," Bowyer told the club's official website.
"He's another player who I feel didn't fulfil his potential here. He went away to Crystal Palace and played towards the back end of the season and just missed out on the playoff final.
"He was hungry to come back here and make his mark, which he was unable to do the first time round. That hunger and desire is what we need."
Marrow made 26 league appearances for Palace.