Claire Jenkins, 36, of Gowerton, was given the fine after leaving bread crumbs on a bench in Oxford Street, during a Christmas shopping trip.
She said: "I found it degrading. I didn't know it was classed as an offence."
Swansea council said a new campaign was responding to complaints people were being "plagued by seagulls".
"Posters have been put up on bins in the city centre advising shoppers of the penalties for littering and our message is that if you don't drop litter you won't face the fine," a spokesman said.

A recording, which a US website claims was Gibson on the phone to Oksana Grigorieva, has emerged. He is allegedly heard verbally abusing her.
Los Angeles County Sheriff's Department spokesman Steve Whitmore said officers will "review" the recordings as part of an investigation.
A spokesman for Gibson has declined to comment on the claims.
"Anything that is connected to the case we will investigate," Mr Whitmore said.
Last week, RadarOnline.com posted a recording which it claimed was Gibson.
A man can be heard telling Grigorieva he thought she was dressing too provocatively using explicit and offensive language.
At one point, Grigorieva accuses the man of "hitting a woman when she's holding a child in her hands". He replies that she "deserved it".
Gibson and Grigorieva are currently battling each other for custody of their 8-month-old daughter.
The Lethal Weapon and Braveheart star previously hit headlines over an angry outburst in 2006.
Then, a leaked police report quoted him making anti-Semitic comments to a Jewish police officer, who was arresting him for drink driving.
A woman has been fined £50 for feeding pigeons in Swansea city centre after a council crackdown.
[NEXT_CONCEPT]
US police have said they are evaluating claims that Hollywood star Mel Gibson allegedly hit his ex-girlfriend.