She joined actress Karen Marie Richardson in a rendition of George Gershwin's Summertime during a performance of Sleep No More.
The 32-year-old blonde, who is married to Carey Hart and had their daughter Willow in June, was enjoying a night out at the show, held in New York's Manderley Bar, when she decided to entertain her fellow drinkers.
The mellow rendition of the number was in stark contrast to her usual aggressive style.
And flustered singer Karen was almost dumbstruck as they took it in turns to sing their lines.
STAR takes to stage at Sleep No More to sing jazz version of Hush Little Baby
The performance was a rare treat for fans of Pink, who has taken a break from music and hasn't released a single or album since last year.
Past audiences at her live shows have heard a very different rendition of Summertime, based on the Janis Joplin Blues-rock version of 1968.
Maybe she's been practising the melodic lullaby on baby Willow at night.