Nukul Seaheng, 28, was nabbed in Ayuthaya on Wednesday and is now back on Phuket, being interrogated by officers at Patong's Kathu Police Station.
The shooting murder of Canadian Francis Alex Degioanni outside his Patong home on his 34th birthday triggered a huge manhunt that led to the jailing of five people for life for their direct involvement, or helping to hire the gunman.
The hunt for the sixth person continued.
A total of 1.5 million baht was paid for the killing. The gunman received 200,000 baht.
The woman who masterminded the crime, Singaporean Diana Goh Li Fun, was sentenced to life in jail on December 29, 2011, although she was free on bail until conviction.
In a final Facebook message before her sentencing, she wrote: ''Hey, be it good or bad friends, always remember once upon a time there was this person you once call(ed) friend!!!
''Forgive and forget, life is short! Never twice but only once in our lifetime!!! Love!!!''
Ms Goh organised the professional ''hit'' on her former colleague on February 19, 2009, in a dispute over property and money.
Eight shots were pumped at Mr Degioanni through the window of his vehicle before the killer and an accomplice fled on a motorcycle.
Mr Degioanni's then fiancee, who was almost in the line of fire when a hired gunman opened up eight times, told Phuketwan later: ''Francis was one of the best people I've ever known in my life. I still miss him and I can never forget what happened. But I hold no grudges.''
Fern Pencharoenwatana has since started a new life in another part of Thailand, away from Phuket and the memories of that evening.
The murder was followed four days later by a second expat murder, causing great concern across Phuket. The killer who battered Ian Kenneth Stuart, 60, to death in his apartment near Tesco-Lotus in Phuket City has never been caught.