The men arrived in Finland in September and were detained on Tuesday in Forssa, southern Finland. They allegedly took part in executing 11 prisoners in June 2014.
The prisoners had been ordered to lean down to the ground and were shot one after another. A video showing the IS executions had been published on the internet.
Finnish police said it was able to track down the information through its own investigations.
Finnish national broadcaster Yle said the two suspects are brothers.
Local media first reported the detention on Thursday morning, and the police confirmed the reports in early afternoon.
In a separate case in the autumn, an Iraqi asylum seeker was arrested. He was also allegedly identified in picture materials available on the internet.
