The amazing 'discovery' was apparently made when they opened up Artyom Sidorkin, 28, to remove what they thought was a serious tumour.
Mr Sidorkin had complained of extreme pain in his chest and had been coughing up blood. Doctors were convinced he had cancer.
"We were 100 per cent sure," said Vladimir Kamashev, a surgeon in Izhevsk in the Urals. "We did X-rays and found what looked exactly like a tumour.
"I had seen hundreds before, so we decided on surgery."
Before removing part of the man's lung, the surgeon investigated the tissue.
"I thought I was hallucinating," said Mr Kamashev. "I asked my assistant to have a look: 'Come and see this – we've got a fir tree here'. He nodded in shock. I blinked three times as I was sure I was seeing things."
Medical staff said that Mr Sidorkin must have inhaled a seed, which later sprouted into a small fir tree inside his lung.
The spruce, which was said to be touching the man's capillaries and causing severe pain, was removed.
"It was very painful. But to be honest I did not feel any foreign object inside me," said Mr Sidorkin. "I'm so relieved it's not cancer."
There was no independent verification of the surgeon's claims