Ms Lee Li Lian of the Workers' Party (WP) was sworn in as the new Member of Parliament for Punggol East on Monday afternoon.
It was the first order of business in what is expected to be a packed week of Parliamentary sittings. Ms Lee is expected to make her maiden speech in the debate on the Population White Paper.
Ms Lee, 34, took her oath of allegiance in English.
Before the sitting began, ministers and MPs were seen shaking hands with Ms Lee to congratulate her. Her fellow WP MPs were also seen snapping photos of her with their mobile phones.
Ms Lee will hold her first Meet-the-People session on Monday night.