The tribunal ruled that two cabinet members of TRT's party paid small parties to run in the April poll to get around a minimum vote requirement in single candidate races.
The TRT was also found guilty of two charges of election fraud.
Shortly before that, the tribunal cleared TRT's rival, the Democrat party, of charges arising from April 2006 polls which were later annulled as fraud allegations amassed.
The judges ruled that TRT had paid an official from the supposedly independent election commission to change party registration information.
"The argument that TRT submitted for their explanation was insufficient," one of the judges said during the hours-long presentation of the verdict.
The judgments against both parties, which some analysts warn could plunge the country into chaos, follow more than a year of upheaval, including last September's coup that toppled Thaksin.
The rulings could significantly alter the political field before crucial national polls expected in December.
TRT officials hoped the Democrat ruling was a signal that they too would escape the election scandal unscathed as tribunal judges began reading out the verdict against their party.
But the ruling was a blow for the TRT, as the judges dismantled the party's defence point by point.
Tensions had risen in the run-up to the court verdicts, which threatened to prolong the political uncertainty that has gripped Thailand since the beginning of last year.