The caution means the 24-year-old soul diva has not been charged, but the incident will remain on the record and could count against her if she is ever charged with a similar offense in future.
Winehouse spent the night at Holborn Police Station in central London after arriving for questioning Friday afternoon.
Police said a 24-year-old woman had been cautioned for common assault. British police do not identify suspects who have not been charged.
A statement released by Winehouse spokesman Chris Goodman said the singer "admitted to a common assault by slapping a man with an open hand and accepted a caution."
"Amy was fully cooperative with inquiries and apologized for the incident," the statement said
A man was quoted in tabloid newspapers as saying Winehouse hit him when he got in her way while she was playing pool at a bar in the Camden neighborhood of London. He said she also head-butted another man who was trying to hail her a cab in the early hours of Wednesday morning.
She was arrested in London in December on suspicion of attempting to interfere with a court case involving Fielder-Civil, but earlier this year police said prosecutors were no longer pursuing the matter.
Fielder-Civil is in jail awaiting trial on charges of perverting the course of justice stemming from a case in which he is accused of assaulting a barman.
Winehouse's musical career has flourished despite her erratic behavior, missed concerts and stints in drug rehab. The Sunday Times newspaper's annual Rich List this week estimated her wealth at $20 million.
Winehouse's spokesman said the singer was "looking forward to continuing her work on new music in the studio."
By Raphael Satter