The 36-year-old woman was attacked while walking along Hoggs Lane, Chorley, towards Bolton Road on Saturday, March 11 at around 2pm, say police.
A passerby driving a blue coloured vehicle on Bolton Road stopped to help the vicitm and give her and her dog a lift home, they added.
A spokesman for Chorley Police said in a post to their Facebook page: "On Saturday March 11 a 36-year-old woman was walking with her dog from the canal towpath along Hoggs Lane, Chorley, towards Bolton Road when she was subjected to an unprovoked assault.
"It is then believed the victim was seen by a passer-by who was driving a blue coloured vehicle on Bolton Road, Chorley who stopped to help the victim and then gave her and her dog a lift home.
"If you witnessed the incident or have any information, or if you are the driver who stopped to take the victim home, please contact PC 8252 Thompson, Chorley Police quoting Crime reference number SC1701879 on 101."