Timan Erdimi, arrived at N'Djamena International Airport early Thursday, where around 50 relatives and supporters were waiting for him, an AFP journalist saw.
Erdimi, the head of the Union of Resistance Forces (UFR), has been living in exile in Qatar for at least a decade.
His armed group attempted to overthrow his uncle, former president Idriss Deby Itno, first in 2008 and again in 2019.
"I am very happy to return home after so many years in exile," said Erdimi, 67, sporting a small white beard and dressed in a traditional gown and white hat.
Erdimi will be a key participant at an "inclusive national dialogue" opening in N'Djamena on Saturday.
The brainchild of junta chief General Mahamat Idriss Deby, the forum will bring together 1,400 delegates from the military government, civil society, opposition parties, trade unions and rebel groups.
Deby has hailed it as a chance for reconciliation in the fractured country, opening the way to "free and democratic" elections within 18 months of the military takeover.
He took power in April last year at the age of just 37 after his father, who ruled for 30 years, was killed during a military operation against rebels.
The "dialogue" should have started in February but was repeatedly delayed as Chad's myriad rebel groups, meeting in Qatar, squabbled over whether to attend.
In the end, around 40 groups on August 8 signed up to a deal, entailing a ceasefire and guarantee of safe passage.
The UFR, one of the signatories, is estimated to have several hundred fighters, based in southern Libya and northern Chad.
The elder Deby, like his nephew Erdimi, hailed from the Zaghawa ethnic group. His successor, the younger Deby, is Erdimi's cousin.
Saleh Kebzabo, a vice president of the forum's organising committee and one-time opponent of the older Deby, said the upcoming talks would facilitate a new constitution which would then be put to a referendum.
But observers say the conference faces major challenges, starting with the pressure of time.
"The timetable for the dialogue, which is supposed to last 21 days, isn't credible," said Enrica Picco of the International Crisis Group (ICG) think tank.
The junta's 18-month deadline runs out in October, leaving scant time for organising a referendum and then elections in this vast, arid country.
Those boycotting the forum include two of the biggest rebel groups and a major political alliance.
Absentees include the Front for Change and Concord in Chad (FACT), which triggered the offensive in the northeast last year that ended in the elder Deby's death. It says the forum is "skewed in advance."
Among political groups, Wakit Tamma, a large coalition of opposition parties and civil society groups, is also snubbing the talks.
"We estimate that 80 percent of (those attending) are close to the junta," said Succes Masra, head of The Transformers party, which is part of the alliance.
It accuses the junta of violating human rights and using the "dialogue" as a springboard for Deby's election candidacy, something that he has previously ruled out.
