The incident happened on the Northern line between Balham and Old Street at around 11.40pm on August 22, 2024.
A woman sat on the tube and a man sat opposite her and began masturbating while staring at her.
Detectives would like to speak to the man in the image who they believe may have information that could help their investigation.
A British Transport Police spokesperson said: “Do you recognise this man?
“Officers investigating an incident during which a man masturbated on board a northern line train last summer have released an image.
“At around 11.40pm on Thursday 22 August 2024, a woman was sat on the tube on the Northern Line between Balham and Old Street when a man sat opposite her and began masturbating while staring at her.
“Detectives are also appealing to anyone who may have experienced similar incidents, to report it.
“All reports of sexual assault are taken seriously and any information provided can help build a bigger picture.
“Anyone who recognises the man is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference number 2400106476.”
“Alternatively you can call Crimestoppers anonymously on 0800 555 111.”
