The Rain Man actor was walking through Hyde Park when he saw Sam Dempster fall to the ground, and he raced to help the sick man during the incident late last month (Apr12).
Hoffman dialled 999 and stayed with Dempster for 15 minutes as paramedics carried out CPR.
Dempster is now recovering from the health scare after undergoing an operation, and has reached out to the actor to thank him for his assistance.
He tells Britain's 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."
Paramedic Martin adds, "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."