The messages justified the attack on July 17, in which seven people died, as an attack on American interests and labelled Manchester United, which was due to book into one of the hotels, as "crusaders".
It said the attack was a warning to Indonesians "against the arrival of the soccer club Manchester United" at the Ritz-Carlton.
Manchester United reject appeals to go ahead with Jakarta match after terror blasts
“These players are Christians, so Muslims should not honour and respect these enemies of Allah," it said.
The posting, which has not been independently verified, was purportedly written by Mohammed Noordin Top, South-East Asia's most wanted man.
It referred to the "American chamber of commerce" as a target, apparently confirming that a breakfast meeting of Western businessmen at the Marriott, hosted by US lobbyist James Castle, was the focus of the attack.
The message dedicated the bombings to Top's dead accomplice Azahari Husin.
Sulistyo Ishak, a police spokesman, said police were investigating the internet posting, which carried Top's name at the end and mentioned al-Qaeda in Indonesia.
An expert on regional terrorism said the language resembles previous claims by Muslim extremist groups and may be authentic.
The splinter group has been considered the most likely perpetrators of the hotel attacks. An unexploded bomb recovered from the scene resembled devices used by the group before and documents seized from members indicated they intended to hit prominent Western targets.
Top is accused of planning four previous attacks in Indonesia that killed more than 240 people. His group claimed responsibility for just one of those strikes — triple suicide bombings in Bali in 2005 — in a similar note.