Political heavyweight Hun Sen, the self-styled strongman of Cambodia, was prime minister for nearly four decades until handing over to his son last year.
Hun Sen provided the billionaire with sanctuary during his exile, naming him a special advisor and allowing him to visit frequently and meet allies in Cambodia, frustrating Thaksin's rivals, who saw it as interference by a neighbouring country.
The activities of Thaksin, a towering figure over Thailand's tumultuous politics, are being closely watched amid expectation he will exert influence on a government led by his family and allies, as he did while in self-imposed exile to avoid jail after being toppled in a coup.
Thaksin was freed on parole on Sunday due to his age and health, with the 74-year-old since seen wearing a neck brace, padded sling and using a wheelchair. A senior official who met him said he was "truly ill".
"Two former prime ministers met and didn't talk about politics," Hun Sen posted on Facebook, with an image of him sitting on a sofa next to a somber-looking Thaksin wearing arm and neck supports.
The full extent of Thaksin's health issues have not been disclosed and critics have questioned whether he is really ill.
Thaksin made a dramatic return to Thailand in August the same day as loyalist Srettha Thavisin being named Thailand's prime minister and Hun Sen ceding power in neighboring Cambodia.
Thaksin was sentenced to eight years in prison for conflicts of interest and abuse of power but was transferred to hospital on his first night in jail with chest pains.
His sentence was commuted to one year by the king and he was paroled after six months.
Some analysts said Hun Sen's visit demonstrated their deep personal connection, one that could strengthen their countries' ties given their influence over their respective governments.
"The visit reflects the politics of the elite in Southeast Asia," Siripan Nogsuan Sawasdee, professor of political science at Chulalongkorn University.
Their relationship could help navigate difficult bilateral issues also, Siripan said, such as joint exploration for offshore hydrocarbons in an overlapping area, which have been stalled for decades.
