X-rays suggested 28-year-old Artyom Sidorkin had a tumour so they operated and were shocked to find a 5cm tree sprout instead.
Surgeon Vladimir Kamashev said: "When I cut I saw tree needles, so, I was very surprised, astonished, thought 'what could it be?'."
Medical staff believe the man somehow inhaled a seed, which later sprouted into a small fir tree inside his lung but biologists are sceptical and the tree has been sent to a laboratory for tests.
Olga Baranova, a local university professor, said: "It is absolutely impossible for a green plant to grow from this small seed inside man's body.
"In order for a plant to start growing from the seed, it needs a lot of conditions, first of all light, then water and certain temperature."