Police said the man exposed himself as a woman jogged past Southsea Rock Gardens.
No contact was made with the woman and she wasn’t hurt, police have confirmed.
A spokesman from Hampshire Constabulary said the incident happened on April 4 at 10am.
He said: ‘Officers have followed a number of lines of enquiry and are now turning to the public for help.
‘We would like to speak to anyone who was in the area at the time and may have seen anything that could assist us with our enquiries.’
Police are keen to speak to a man who was pictured in the area at the time.
The man, photographed, is white, roughly 5ft 10in tall, slim and with receding hair.
He also has the word ‘love’ tattooed in faded lettering on the knuckles of one of his hands.
Anyone with information should call 101, quoting the reference number 44210126334.