The attack took place on morning of Tuesday 10 May on Fish Street Hill by Monument Tube Station.
A spokesman for British Transport Police said: “We are currently investigating after a man was assaulted at Monument station at around 7.40am on Tuesday, 10 May.
“Enquiries into the incident are ongoing. Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference 343 of 12/5/16.
"Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.”