The suspect was identified as Maduemang Masae, a 39-year-old security volunteer in the southern border province of Narathiwat, according to Pol Lt Gen Anuchai Lekbamrung, commander of Provincial Police Region 4.
The suspect was flown from Bangkok to Udon Thani in a police plane on Friday night, and was being questioned at the 24th Border Patrol Police Unit before being moved to Udon Thani Central Prison.
Pol Lt Gen Anuchai said Mr Maduemang admitted to the Jan 21 shooting and said he was paid about 8,000 baht.
Mr Kwanchai is the chairman of the Rak Udon faction of the United Front for Democracy against Dictatorship in the northeastern province and one of the most outspoken red-shirt leaders. Police said Mr Maduemang told investigators that he decided to join the operation involving another six people after being approached by a "well-respected elder" who had helped him previously.
"The investigation is in line with the testimony of the suspect who was not hesitant to confess," Pol Lt Gen Anuchai said. "Police will investigate further and search for the other six accomplices."
The officer did not divulge any information on the "well-respected elder".
A week before Mr Kwanchai was attacked at his home in Udon Thani on Jan 21, a message appeared on a social network saying that hitmen in southern Thailand had offered a 1.5-million-baht bounty for the murder of the hardline red-shirt member.
The attack occurred about 10.30am while Mr Kwanchai was reading a newspaper in his home. He was hit many times in an arm and leg. Police recovered about 30 spent M16 bullet shells near a fence located about 200 metres from the house.