The former Atomic Kitten singer, 36, said she felt “violated” and planned to report the “vile” man to the police.
The mother of four was travelling home to south-east London.
Sharing the snap with her 162,000 Twitter followers, she said “this vile excuse of a man” pushed himself against her during a journey from London Bridge.
She tweeted: “I thought it was a bag until I felt him heavy breathing down my neck. I am appalled and calling the transport police when I get off!” A second tweet added: “Honestly I am shaking, feel so violated now! But more than anything I am so angry!!”
Fans rushed to support her. One said: “You’re doing the best thing by reporting it and luckily photographing it. Did the train have CCTV? That would be a bonus.”
Another added: “This needs to be stopped… no one should have to go through that anytime anywhere!” A third wrote: “I think there should be a carriage for females only as there is no way to police this issue.”
Ms Hamilton is well known for sharing her opinions on social media and her blog.
She recently wrote about the stigma surrounding older mothers and her experience with post-natal depression.
Last night a spokesman for British Transport Police said: “Officers are making inquiries into this incident as part of an investigation.
“We take every report of unwanted sexual behaviour with the utmost seriousness.
“Through our Report It To Stop It campaign we are encouraging victims to come forward to tell us what happened.”