In a recent interview, the 25-year-old described her former colleague as a "lovely girl", saying that she admires Momsen's talent as both an actress and musician.
"She's so talented, and anyone I know that's heard her music is a huge fan," Meester told E! News. "You can't help it. She's also an incredible actress and just a lovely girl."
Last month, Momsen said that she had quit acting in order to focus on her music career.
Meester, who enjoys a career as both an actress and musician herself, revealed that she could never pick between the two professions.
"Both is where it's at," Meester said. "If I had to choose, I think I'd be very unhappy."
Momsen was dropped from Gossip Girl in May after being placed on an "indefinite hiatus" in November 2010.