Tidiane Cheik Diaw, 27, an illegal immigrant who had met Olsen in a nightclub hours before her death, was convicted of homicide.
The judge rejected the prosecution's request for the charge to be considered as having been aggravated by acts of cruelty.
Diaw, who can appeal against the verdict at least once, was arrested on January 14th, five days after the discovery of Olsen's body.
The prosecution matched his DNA to traces recovered from the dead women and from a condom and a cigarette butt found in the flat.
A post-mortem concluded that Olsen, 35, died as a result of strangling but she also suffered a double fracture of the skull.
The prosecution's case was that the couple had fought after having consensual sex.
An artist and event organizer, Olsen had been living in Florence for several years and was a well known figure in the American expatriate community in the Tuscan city.
She had moved from her native Florida to be close to her art teacher father Walter Olsen and was regularly seen around the city's historic centre with her pet beagle.
Just after her death, friends depicted a vivacious and fun-loving character while her father voiced his heartbreak at losing “a beautiful and creative young woman with a happy, exuberant and generous soul, who loved her life in Florence.”
Olsen's family were in court on Thursday to hear the verdict.