He arrived on Tuesday morning in a private jet, ahead of a vote for the next Thai leader - the frontrunner is from his Pheu Thai party.
Thailand's former Prime Minister, Thaksin Shinawatra, has been jailed upon returning to the country after 15 years in exile.
But many believe he has struck a deal that will keep him from serving more than a short period in prison.
He arrived on Tuesday morning in a private jet, ahead of a vote for the next Thai leader - the frontrunner is from his Pheu Thai party.
He was then sentenced to eight years, on former criminal convictions, which he says were politically motivated.
Thaksin, Thailand's most successful elected leader, has long been feared by conservative royalists, who have backed military coups and contentious court cases to weaken him.
But now the brash, politically ambitious telecoms tycoon is back, years after he was deposed by a military coup. He landed in Bangkok's main airport to cheers from hundreds of loyal supporters who had gathered overnight to see him. Flanked by his two daughters and son, he emerged briefly from the airport terminal and paid his respects to a portrait of the king and queen, BBC Reports.
The 74-year-old was immediately taken to the Supreme Court where he was sentenced to eight years on three former convictions, and then to Bangkok Remand Prison.
Prison authorities there say he will be kept in a wing with specific medical equipment, given his advanced age. He will also immediately undergo a 10-day quarantine - the first five days of which he will be confined to his room, authorities said.
It has been speculated that Thaksin will seek a royal pardon, and prison authorities on Tuesday said he would be able to submit a petition from jail immediately. The process can take one to two months.
Hundreds of "red shirt" loyal supporters had gathered at Bangkok's Don Mueang Airport on Tuesday morning to welcome the former leader's return.
Samniang Kongpolparn, 63, had been waiting since Monday evening to see Thaksin. She, like many of the other supporters, had travelled from Surin province in the northeast, the stronghold of Thaksin's party in past decades.
"He's the best prime minister we've ever had. Even though I won't get to see him today, I still wanted to come to show him support," she said. "I'm ok with them reconciling with the pro-military government, or else we're stuck with the senators. We don't want that."