Francis Alex Degioanni, a Canadian national aged 34, was gunned down in February 2009 in a car outside his home.
Mr Degioanni had reportedly been doing business in Phuket – selling condos to foreigners –for five years at the time of his death.
Police announced they detained a suspect named Nukul, also known as Nu Sae-heng, 28. He is originally from Phetchaburi and was arrested in Ayutthaya on a Phuket arrest warrant.
He is understood to be one of the key players allegedly involved in the murder of Mr Degioanni.
Nukul is now in custody of Kathu Police.