Thaksin was ousted from power in a 2006 military coup and has been at the heart of 17 years of on-off turmoil in Thailand, influencing politics from exile.
Thaksin, a former telecoms tycoon and owner of Premier League football club Manchester City, was accused by the military of corruption, cronyism and abuse of power.
He was sentenced to jail in absentia in 2008. His sister, Yingluck Shinawatra, was ousted as prime minister in another military coup in 2014.
Thaksin received a rock star reception as a fugitive on his return to Thailand in August, with crowds cheering his arrival by private jet and policemen saluting him before escorting him to court and then jail.
The same day, his ally and real estate mogul Srettha Thavisin was elected prime minister by parliament.
Thaksin has repeatedly denied still being in politics and says he is retired.