The 35-year-old victim was assaulted at around 11.45pm on Saturday in King Street, Musselburgh.
The attacker is described as a white male of average height, with fair or blonde hair who was wearing jeans and a dark top.
He spoke with a local accent.
A police spokesman said: “Inquiries are ongoing to establish the exact nature of the assault, and we are appealing for any witnesses who were in the area to come forward.
“Anyone who was in the area of Newbigging and Pinkie Road at the time of the assault, who noticed any suspicious activity, should contact police.
“Anyone else with any other information that can help our inquiries should also get in touch.”
Anyone with any information is asked to contact Lothian and Borders Police on 0131 311 3131, or Crimestoppers in confidence and complete anonymity on 0800 555 111.