"The attack that targeted a nightclub for homosexuals in Orlando, Florida and that left more than 50 dead and many wounded was carried out by an Islamic State fighter," it said in a terse statement quoting a "source".
Amaq, the IS-linked news agency, gave no further details. US media reported that the gunman -- widely identified as 29-year-old Omar Mateen, an American citizen of Afghan descent -- had pledged allegiance to the jihadist group IS.
Fifty people were killed, in addition to the shooter, and 53 wounded in the worst mass shooting in US history, the mayor of Orlando Buddy Dyer said earlier on Sunday.
US officials were probing whether the suspect had radical Islamic "leanings". US President Barack Obama said the FBI was investigating the killings "as an act of terrorism".