Thai police spokesman Lieutenant-General Ronnarong Youngyuen said the relevant authorities had nothing on Pathmanadan.
"If we'd arrested him, we would have made good publicity out of it"
Thai police spokesman
"I've checked with [the] related police bureaux – the Immigration Police, the Metropolitan Police and the Special Branch," he told Reuters.
"There has been no report of a Tiger rebel arrested in Bangkok," he said, adding, "If we'd arrested him, we would have made good publicity out of it."
Ronnarong said Thai police did hand over three Tamil Tigers to Sri Lanka last month after the rebels served jail terms for arms smuggling following their arrest four years ago.
On Monday a posting on the Sri Lankan Defence Ministry website said: "reliable sources from Thailand reveal that LTTE's chief for cross border terrorist activities, Kumaran Padmanadan, alias 'KP' has been arrested in Bangkok".
In Thailand the English language Bangkok Post also reported Pathmanadan's arrest in the Thai capital.
Known as the Tiger's money man, Pathmanadan has been on Interpol's most wanted list for a number of years, and implicated in several assassinations.
Polcie have searched for him under a number of aliases in Johannesburg, Yangon, Singapore and Bangkok.
They believe he had bank accounts in London, Frankfurt, Denmark, Athens and Australia, and has more than 200 passports.
The LTTE has been fighting government forces since the early 1980s, seeking to carve out a separate homeland for the ethnic Tamils.
About 70,000 people have been killed in the conflict to date.