He will now compete with the likes of Ben Botica and Matt Hopper to take centre stage, while Tom Casson, George Lowe and Jordan Turner-Hall also remain options.
"Given our injury situation and the time of the season, we are very fortunate to sign a player of Tim's experience," said Harlequins director of rugby Conor O'Shea
"We are looking forward to welcoming him to the Harlequins family."
Molenaar has not featured in any of Gloucester's Heineken Cup matches this campaign and is there for eligible if Harlequins opt to add the New Zealander to their European squad.
The Twickenham club languish bottom of Pool Four, having lost their opening two fixtures to the Scarlets and ASM Clermont Auvergne.
They must now win their back-to-back matches with Racing Metro in December to keep any realistic hopes of qualifying for the knockout stages alive.
Meanwhile Gloucester director of rugby Nigel Davies has paid tribute to Molenaar's contribution in the West Country.
"Tim has been a popular member of the squad in his time here and, even though he has not had many chances in the first team recently, has been a good professional," added Davies.
"With Harlequins having some injuries in their squad, there is a chance for Tim to get some more playing time at the Stoop, so we have agreed to release him.
"We would like to think him for all his efforts over the past four seasons and wish him all the best for the future."