The Luck and Rain Man actor happened to be passing when Sam Dempster collapsed.
The 74-year-old Hollywood legend called 999 before staying by his side for 15 minutes as paramedics carried out CPR.
Dempster told The Sun: "I want to say 'thank you' to Dustin Hoffman. He saved my life.
"I have no memory of what happened. The paramedics told me I had been saved by Dustin Hoffman. It's unbelievable."
Hoffman, who has a home in Kensington, told doctors that the runner had been frothing at the mouth before staggering to a halt.
Paramedic Martin Macarthur, 45, recalled: "Dustin was fascinated. He seemed impressed we'd got this guy back so quickly.
"When we were carrying the patient into the ambulance, he walked up and said 'Great job, guys'. He really appreciated what had happened."
Watch Dustin Hoffman accepting his 'Best Actor' Oscar for Kramer vs Kramer below: