The Isis-linked Aamaq News Agency said the attack was carried out by a "soldier of the caliphate" who opened fire from an automatic rifle in response to the orders of Isis leader Abu Bakr al-Baghdadi.
Turkish media reports had earlier said that the country's authorities believed Isis was behind the attack and that the gunman, who is still at large, comes from a Central Asian nation and is likely to be either from Uzbekistan or Kyrgyzstan.
According to the Hurriyet and Karar Turkish newspapers, police had established similarities with the high-casualty suicide bomb and gun attack at Istanbul's Ataturk Airport in June and were investigating whether the same Isis cell could have carried out both attacks.
The nightclub gunman killed a policeman and another man outside the Reina club in the early hours of 2017 before entering and firing at an estimated 600 people partying inside with an automatic rifle.
Nearly two-thirds of the dead in the club, frequented by local celebrities, were foreigners, the Anadolu Agency said. Many of them were from the Middle East.
The mass shooting followed more than 30 violent acts over the past year in Turkey, which is a member of the Nato alliance and a partner in the US-led coalition fighting against Isis in Syria and Iraq. The country endured multiple bombings in 2016, including three in Istanbul that authorities blamed on Isis, a failed coup attempt in July and renewed conflict with Kurdish rebels in the south-east of Turkey.
Isis claims to have cells in the country. Analysts believe the group was behind suicide bombings last January and March that targeted tourists on Istanbul's popular Istiklal Street, as well as the attack at Ataturk Airport in June, which killed 45 people.
In December, Isis released a video purportedly showing the killing of two Turkish soldiers and urged its supporters to "conquer" Istanbul. Turkey's jets regularly bomb the group in the northern Syrian town of Al-Bab. Turkish authorities have not confirmed the authenticity of the video.
Prime Minister Binali Yildirim said the attacker left a gun at the club and escaped by "taking advantage of the chaos" that ensued. Some customers reportedly jumped into the waters of the Bosporus to escape the attack.
