"As the UPFA did not make any claim for the opposition leader's post, I like to inform the house that Mr Sampanthan the leader of the TNA has been recognised as the leader of the opposition," Speaker Karu Jayasuriya told parliament today.
The United People's Freedom Alliance (UPFA), which previously made up the Opposition, has joined Prime Minister Ranil Wickremesinghe's United National Party (UNP) in government.
The TNA, which fought election under the symbol of Ilankai Thamil Arasu Kadchi (ITAK), won 16 seats.
Sampanthan, 82, becomes only the second Tamil to lead the opposition in the history of the island. In 1977 then main Tamil United Liberation Front (TULF) leader Appapillai Amirthalingam became the opposition leader.
Earlier this week Sampanthan had said that his party would push for justice and greater autonomy for the ethnic minority, who say they suffer widespread discrimination on the Sinhalese-majority island.