Piyasena Gamage was sworn in place of former parliamentarian Geetha Kumarasinghe as a UPFA MP before Speaker Karu Jayasuriya this morning.
Gamage was named for the seat, after the Supreme Court held that Kumarasinghe could not hold the post of an MP as she was a dual citizen.
The new MP sat on government benches after taking oaths.
Joint opposition MP Priyankara Jayaratne invited Gamage to the opposition while he was walking towards his seat.
"Come to our 'side and sit with us," Jayaratne was heard saying. (Yohan Perera and Ajith Siriwardana)