The Paris-based Reporters Without Borders organisation said the al Qaeda-affiliated Islamic State of Iraq and the Levant (ISIL) seized Yasser Faysal al Joumaili while he was on a reporting trip in northern Syria's Idlib province on December 4.
Mr Joumaili was then executed and his body later arrived in Turkey, although the exact circumstances of his death were unclear, the watchdog organisation said in a statement.
Mr Joumaili was from the Iraqi city of Fallujah and had three children, the group added. He worked for Reuters in Iraq from 2003 to 2009.
Reporters Without Borders said Mr Joumaili was the 20th professional journalist and eighth foreign journalist to die in the Syrian conflict, which started in March, 2011, as a peaceful protest movement and slid into civil war after a crackdown on protesters.