Police say around 9:40 p.m. Wednesday, a woman was walking along a path in the 600-block of Osgoode Drive when she saw a man on a mountain bike approaching her.
Officials say she stepped off of the path to allow the man to pass her.
Then, 10 minutes later, police say the same man returned on the bike and approached the woman from behind.
Police say the man reportedly sexual assaulted the woman and left the area. They say the woman was not physically injured.
Officials describe the suspect as a man in his 20s or 30s with tanned skin, short dark hair and a thin build.
He was wearing a dark hooded sweater and dark jeans, and riding a dark-coloured mountain bike.
Anyone with information is asked to call the London Police Service at (519) 661-5670 or Crime Stoppers at 1-800-222-TIPS (8477).