Michael Boatwright (61) was found unconscious in a room in Palm Springs in February.
After police arrived, he was transported to the Desert Regional Medical Center, where he woke up with amnesia, but identified himself as Johan Ek.
Hospital officials said Boatwright may have been in town for a tennis tournament in the Coachella Valley.
He was found with a duffle bag of exercise clothes, a backpack and tennis rackets.
He also carried four forms of identification – a passport, a California identification card, a veteran's medical card and a Social Security card – all of which identified him as Michael Thomas Boatwright.
In March, doctors diagnosed Boatwright with Transient Global Amnesia – a condition triggered by physical or emotional trauma.
The rare mental disorder is characterised by memory loss, "sudden and unplanned travel", and possible adoption of a new identity. A local newspaper located his sister in Louisiana, but she was unable to shed any light on what had happened.
"I haven't talked to him in years. He just disappeared," Michelle Brewer said.
Brewer said she last spoke to her brother 10 years ago.
Swedish public records show Boatwright lived in the Nordic country between 1981 and 2003.
Boatwright doesn't recall his son and two ex-wives either.