Derek Hill, 57, attacked the woman after barging his way into a toilet cubicle..
Hill - a crew commander at Kelso Fire Station - forced his tongue into his victim’s mouth while reaching under her skirt and pulling her underwear down to her knees.
He also fondled the woman’s genitals while unbuttoning his shorts during the sex assault in the Borders town in 2020.
The victim, who cannot be identified due to legal reasons, managed to flee the toilet area but did not confide in anyone until one week later.
Hill denied the attack but was found guilty of sexually assaulting the woman by a jury following a four-day trial at Edinburgh Sheriff Court.
The married father-of-two has been jailed by Sheriff Daniel Kelly who told him the attack had had a “devastating effect” on the woman.
The sheriff added the incident had “affected her mental health” and she has since been forced to move away from the area.
Sheriff Kelly said: “Given this is a serious sexual assault, in my view the only appropriate disposal is a sentence of imprisonment.”
Hill was jailed for three years and placed on the Sex Offenders’ Register for an indefinite period.
He was also made subject to a 10 year non-harassment order banning him from contacting the woman in anyway.
The victim, who is in her 30s, attended the party to celebrate Hill serving 30 years service in Kelso on July 18, 2020.
She said she arrived at the event held at a car wash business owned by a fighter colleague of Hill at around 2pm.
When she tried to leave the toilet cubicle she found Hill blocking the door.
The woman said Hill “pushed the door open” forcing her back inside before he stepped into the cubicle and locked the door.
She said Hill “forcefully” kissed her on the mouth before he reached under her skirt and “pulled down my underwear”.
The woman added: “He was touching me under my dress. He pulled down my underwear and he was unbuttoning his shorts with one hand and touching me with the other.”
The court heard the woman did not say anything at the party as she “didn’t want anyone to know” about the attack.
She said: “I think I was in shock at the time and tried to act as normal as I could.”
She later confided in a friend before reporting the assault to Hill’s superiors at Kelso fire station.
An internal investigation was set up and Hill, from Kelso, was suspended.
Hill also gave evidence and said the victim sat on his lap three times and had asked him for a kiss in return for a gift she had bought him.
He admitted “overstepping the mark” by going into the cubicle and kissing the woman on the neck but denied sexually assaulting her.
Following the guilty verdict Area Commander Stephen Gourlay said: “The Scottish Fire and Rescue Service demands the very highest standards of all our staff and one of our core values as an organisation is respect. Following an internal investigation, Hill was ultimately dismissed in October 2020."