The verdict brings to a close a long-running and controversial labour dispute between the reporters and their employer ITV.
The broadcaster - controlled by Shin Corp, which is now owned by members of Thaksin's family - sacked the reporters and producers in February 2001 after they attempted to form a union to protest against political interference.
The employees took ITV to court and won their case, then won an appeal filed by the broadcaster, and finally a verdict from the country's high court.
"The supreme court said in its ruling that ITV's appeal was groundless and its order to sack the 21 reporters was unlawful. Thus the court upholds the appeals court order," the Central Labour Court official said in conveying the higher court's verdict.
Under the ruling, the group will now be paid wages owed since the date they were dismissed, former presenter and producer Karuna Buakamsri said, adding that the decision was a victory for press freedom in Thailand.
"We are very glad. Some of us openly wept" when the verdict was handed down, Buakamsri, one of the 21 who were sacked, said.
"This ruling should boost morale for any people who are victims of injustice. No matter what, we have to be confident in justice"
sacked ITV presenter and producer
"This ruling should boost morale for any people who are victims of injustice. No matter what, we have to be confident in justice."
Karuna said all 21 would return to work at ITV headquarters on Wednesday.
"We will report to work at 9am, and we will see what the ITV management's attitude is," she added.
The station had yet to provide an official reaction to the verdict.
Karuna said she and her colleagues expect ITV to pay 40 to 50 million baht ($1-1.3 million) in back wages.
During the campaign for the January 2001 elections which Thaksin's Thai Rak Thai party won in a landslide, the ITV journalists complained they were being forced to give a positive spin to Thaksin's policies.
Concerned about editorial interference at a station that had been noted for its lack of bias, they attempted to form a union, but were sacked a day after their first meeting.
Thaksin has had several brushes
with the media in recent years
ITV's management alleged that some journalists had violated company regulations, while others had to be laid off due to financial constraints - claims rejected by the sacked staff.
The row was Thaksin's first serious brush with the media, but since then there has been a string of flare-ups, including moves to deport two foreign Bangkok-based journalists and a $10 million lawsuit against a media-rights campaigner.
Thaksin's Thai Rak Thai won a landslide re-election last month.