Danai Raiwech, 48, was arrested at his home in a Bangkok suburb on Thursday after police searched his property in connection with a $325,000 heist at a jewellers.
His wife was found wearing items of stolen jewellery, but during the search officers also uncovered bags full of underwear.
"He confessed that he obsesses over underwear and uses it for masturbation," Police Colonel Napanwut Liamsanguan told AFP.
"He collected underwear for many years, stealing them from houses or apartments three or five each time. I stopped counting at 2,000 pieces of underwear but there must be more than 5,000 of them I'm sure," he said.
His wife was apparently aware of his problem but he had refused to see doctor, the policeman added.
In January officers found more than a thousand items of underwear in the trunk of Danai's car after they arrested him for burglary, while several thousand more were uncovered at his house during a subsequent search.
He was bailed on that occasion after admitting stealing and collecting women's underwear since the age of 18.
Napanwut said he would be charged for the jewellery robbery but it would be difficult to prosecute for the underwear theft because the owners must first file a complaint to police.