Formisano, who works at the Institute of Physics and Interplanetary Science in Rome, says measurements from the European Space Agency's Mars Express spacecraft, which has been orbiting the red planet since December 2003, suggest there is far more methane there than previously thought.
The scientist believes Martian soil microbes are the most likely source.
He told New Scientist magazine, "Until it is demonstrated that non-biological processes can produce this, possibly the only way to produce so much methane is life. My conclusion is that there must be life in the soil on Mars."
Formisano analyzed data from an instrument on board the orbiting probe.
He will present his findings at a conference this week.
[via Ian Sample and the Guardian Unlimited]