An Islip man was seriously injured when he jumped out of moving vehicle his girlfriend was driving on Sunrise Highway in Bohemia Sunday afternoon, police said.

Christopher Ryan, 34, of Islip, was a passenger in a 2009 Nissan Altima that was traveling west on Sunrise Highway between exits 50 and 49 about 2:10 p.m. Sunday when, “for unknown reasons” he jumped out of the moving vehicle, Suffolk police said in a news release.

Ryan’s girlfriend, Alexandria Marston, 26, of Islip, was driving the Nissan and stopped a short distance away, police said. Another motorist traveling behind the Nissan witnessed the incident and stopped and shielded Ryan from further injuries, police said.

Ryan was taken to Stony Brook University Hospital by the Lakeland Fire Department, where he was treated for serious, but not life-threatening, injuries, police said.

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Marston and the man’s 9-year-old son, who was a passenger in the Nissan, were not injured.

Marston could not be reached for comment Sunday night.