Martin O’Neill inappropriately touched the woman at Glasgow’s Bar Home on April 10, 2022.
The married 41-year-old was inadvertently captured on camera after a colleague filmed O’Neil due to how drunk he was.
It lead to him being reported to his Police Scotland bosses.
Shamed O’Neill appeared at Glasgow Sheriff Court where he pleaded guilty to a charge of sexual assault.
It emerged O’Neill – who only joined the force in 2019 – resigned before appearing in the dock.
He was put on the sex offenders list and will be sentenced at a later date.
The court heard how the night out had occurred at the bar in Glasgow’s Merchant City.
Prosecutor Caitlin McAllister said: “O’Neill was seen by colleagues placing his hand on (the woman’s) thigh for a minute or two.
“Other officers saw him touching her there at various points during the night.”
O’Neill was described as “very drunk” in the pub.
Miss McAllister said: “At 11.45pm an officer recorded O’Neill.
“While doing this, he caught O’Neill using his left hand to cup the victim’s upper right thigh towards her buttocks a few times.”
Some of his concerned colleagues messaged each other about how uncomfortable the victim looked.
Describing another incident that night, Miss McAllister said: “O’Neill was seen reach over with his hand and try to slap the victim on her buttocks.
“He missed and made contact with her rear upper thigh area.”
The victim later confronted O’Neill, who apologised for his conduct.
The matter was then reported leading to O’Neill being charged.
Christopher Shaw, defending, told the court: “He has tendered the plea on the basis he was reckless, drunk and overfamiliar.”
Sentencing was deferred pending background reports until September by Sheriff Gerard Bonnar.