The 47-year-old woman was walking along the canal between Swinton and Mexborough at around 12 noon on Saturday, May 15, when she was approached from behind by a man who grabbed and sexually assaulted her.
The attacker was interrupted by a man who was passing by and chased him off.
A South Yorkshire Police spokesman said: "We are keen to identify the good Samaritan who helped the woman, as he later returned and kindly gave the victim a lift home. He could provide key information to assist our investigation.
"The suspect is described as white and wearing a black hoody. If you witnessed anyone acting suspiciously along the canal path on Saturday, or you are the good Samaritan we are trying to trace, please get in touch.
“Thankfully, the woman wasn’t physically injured but was left shaken by this incident. We’d really appreciate any assistance to help progress our inquiry.”
Anyone with information can call South Yorkshire Police on 101 quoting incident number 248 of May 16.