The shameful Snapchat message that cost this policewoman two jobs.

A US police officer has been fired from one of her two stations after she posted a racial slur on social media.

Melissa Adamson, from Pennsylvania, shared a Snapchat in full uniform with a caption that read: “I’m the law today n—-a.”

The image lost Adamson two roles for behaviour the McKeesport Mayor Michael Cherepko called “absolutely unacceptable”.

Cherepko released a statement on Facebook that said Adamson had lost her position as part-time employee in the McKeesport district.

“This post displays a degree of conduct and character that is far different from what I would expect from an officer in this city,”he said.

“It is absolutely unacceptable. Without hesitation, my office and the police chief’s office immediately concluded that this officer’s action will not be tolerated in the City of McKeesport. She had been relieved of her duties, and her employment has been terminated."

After she was sacked in McKeesport, the officer resigned from a second part-time position she held in the Pennsylvanian area of Versailles, local news station WTAE reports.

Adamson told WTAE she believed the image had been circulated maliciously by a former officer.

"An altercation happened between me and a former police officer, and he’s bringing up his dirt, ruining my career, which has been done," she said.

Adamson apologised for her conduct and said hadn't intended to cause racial offence.

"I can’t express how sorry I am for how I made you feel – emotionally, physically or what not. It wasn’t my intention," she said.