A young woman has been sexually assaulted as she took an evening walk along a riverbank in southwest Sydney.


Police say the 20-year-old was walking along a bike path by the Cooks River at Canterbury about 6.50pm (AEDT) on Tuesday when she realised she was being followed by a man.


She was pushed down from behind and sexually assaulted.


The woman's screams were heard by a group of people in a nearby backyard, who yelled at the man until he took off.


The woman was taken to Royal Prince Alfred Hospital.


Detectives say they are looking for a man described as being of African appearance with a dark complexion, aged in his mid-20s with a large or muscular build and short black hair.


He is believed to have been wearing a light blue singlet and long basketball shorts at the time of the attack.


Anyone with information can call Crime Stoppers on 1800 333 000.


AAP