Workers' Party member Lee Li Lian took her oath as the new Member of Parliament for single member constituency Punggol East Monday.
That brings to seven the number of MPs from the opposition party, aside from its two Non-Constituent MPs.
In a four-cornered race, Lee won the Punggol East by-election last month with 54.5 per cent of the votes, defeating People's Action Party candidate Dr Koh Poh Koon who got about 42 per cent.
The result was seen as a rebuke to the ruling party amid citizens' unhappiness over rising cost of living and the influx of foreigners in recent years.
Lee rode on the back of campaign promises that she would fight to make the voices of young couples and the elderly heard in Parliament.
Lee will be holding her first meet-the-people session Monday night at Blk 135 Rivervale Street.