Police are seeking information after a female cyclist was slapped on the bottom by a man in a car, causing her to lose control of her bike and break her collarbone.

The 52-year-old Geelong woman was slapped while riding her bike behind a friend along Horseshoe Bend Road in Torquay at about 8.50am on Saturday.

Police say a man travelling in a dark-coloured 4WD leaned out of the front passenger window and slapped the woman on the backside and then slapped the woman's friend.

The woman lost control of the bike and was later taken to Geelong Hospital with serious injuries, including a broken collarbone.

The offender is described as a teenager with brown, shoulder length hair and wearing a dark-coloured top.

Police say the car involved was a 4WD, possibly a Mercedes with a registration similar to TBU 683. It was last seen turning into Quay Estate towards Fischer Street.

Police are urging anyone with information to contact Crime Stoppers on 1800 333 000 or www.crimestoppers.com.au.