Andreas Loverdos, a veteran politician and former minister with the socialist PASOK party, has officially joined the ruling center-right New Democracy party after a meeting with Prime Minister Kyriakos Mitsotakis.
The move, which had been widely anticipated, marks a significant political realignment.
Mr. Loverdos, who described himself and the prime minister as "moderate, reformist, and liberal," is expected to run for a parliamentary seat in a competitive Athens district in the next election.
Mr. Loverdos, a 69-year-old constitutional law professor, left the PASOK party nearly two years ago, citing disagreements over its political direction.
He was recently cleared of any wrongdoing in the high-profile Novartis pharmaceutical scandal.
His inclusion is seen as part of Mr. Mitsotakis's ongoing strategy to broaden his party's appeal by attracting prominent figures from the center-left.
By Angelos Moschovas