"Yes, I am coming to work full time and will chair every cabinet meeting," said Thaksin on Tuesday.
Thaksin leads a government without a parliament and unable to carry out legislative duties after a court ruled his recent election victory invalid
Thaksin had handed over many of his day-to-day duties to his deputy, saying he was "taking a break" in the interests of reconciliation after months of street protests against his premiership.
Opposition leaders said Thaksin's return would create more political and legal turmoil.
"If Thaksin returns to work as prime minister, political tension will come back again," said Suriyasai Katasila, a spokesman for the People's Alliance for Democracy.
Thaksin attempted to end the protests by calling a snap election in April.
The poll was boycotted by opposition parties, and millions of Thais marked the abstention box on the ballot paper as a protest against Thaksin.
Abstentions and the boycott meant that in some constituencies a winner could not be certified, and the courts declared the result of the poll undemocratic and unconstitutional.
Also on Tuesday, suspected Muslim insurgents shot and killed two Thai border police in the south of the country.