Between 6.45pm and 6.55pm on June 8, the man allegedly touched the woman at the junction of Holmesland Lane and Winchester Street.
Hampshire police have now released an image of a man they would like to speak to in connection with the incident.
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He is white, aged either 18 or 19, of average build and blonde hair.
A spokesperson for Hampshire police said: “We believe the man in this image can help us with our investigation and would urge him, or anyone who knows his identity, to get in touch with us.
“We’d also like to hear from you if you were in the area at the time.
“Do you have any footage which may assist in our investigation, including dash cam, mobile phone or doorbell camera?
“If you can help us, please call 101 and quote incident number 44250250982.”
