The troubled singer was performing at the Glastonbury Festival in front of 80,000, when she punched a spectator in the first row during "Rehab."
She had stepped down into the pit during the performance. After lashing out at the fan, she was gently escorted back to the stage by security staff.
Winehouse got violent because someone grabbed her breasts, said concert organizer Michael Eavis.
Her appearance at the festival seemed doubtful after she was hospitalized for collapsing in her London home earlier this month.
In newspaper interviews published last weekend, Mitch Winehouse, the singer's father, said his daughter had damaged her lungs and developed early-stage emphysema as a consequence of allegedly smoking crack cocaine and cigarettes.
The performance was part of Nelson Mandela's 90th birthday celebration. Winehouse was just one of many stars performing at the event.
