Sam Dempster, 27, was jogging in London's Hyde Park when he collapsed and began frothing at the mouth. Hoffman was walking by and stopped to help.
Hoffman waited with Dempster for paramedics to arrive and described to them what had happened. Dempster reportedly suffered a cardiac arrest.
"It is essential to get a history of what has happened, and he witnessed the jogger go down. He was the main witness and saw him stagger and go down," said one of the responding paramedics. "He stayed on the scene for the whole duration of the resuscitation which was about 15 minutes. He was very concerned."
The paramedic said the 74-year-old actor told medics, "Great job, guys" after they resuscitated Dempster with a defibrillator.
Dempster initially credited Hoffman with having summoned an ambulance, but the London Ambulance Service said someone else had made the emergency call.
Hoffman has a home in London.
The Associated Press contributed to this report.