Bangkok police have charged the man with illegal possession of weapons and officials say he was involved in the attack.
However, they say he is not the man seen on CCTV footage leaving a bag at the Erawan Brahma Shrine before the explosion.
The bomb tore through the crowded shrine on August 17, killing 20 people and injuring more than 100, mostly tourists.
National police spokesman Prawut Thavornsiri said officers believed the suspect was part of a gang that helped illegal migrants obtain counterfeit documents and the bomb attack was in response to the recent crackdown by Thai authorities.
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"They (the gang) are unsatisfied with police arresting illegal entrants," he was quoted as saying by local media.
"It's a network that fakes nationalities and sends them (illegal migrants) on to third countries," he said.
Media reports yesterday had said the suspect arrested was a Turkish national as a Turkish passport was found with his photo in the room.
The Turkish Embassy also denied that the person was a citizen of Turkey, Prawut said.
The man has refused to answer interrogators' questions on whether he played any role in the blast, he said.
Prawut said the suspect's nationality was yet to be confirmed.
The suspect was arrested from an apartment in an eastern Bangkok suburb where police found huge bomb-making materials and several forged passports.
Prawut said several embassies had offered help to Thai authorities to check the background and nationality of the suspect.
He said it was believed that many other people, some of whom were likely to be Thai nationals, were involved in the attack.