Police said the woman was running along the Todd River, near the Stott Terrace Bridge about 3.45pm.
Police said a young man travelling on a black bike was travelling nearby, and that he allegedly indecently assaulted the woman.
The youth has been described as tall with tanned skin.
He was said to be wearing a black shirt with the white numbers “2000” on the back.
Anyone with further information has been urged to come forward to police.
