The Workers' Party collected one set of nomination papers from the Elections Department on Friday afternoon.
Hougang MP Png Eng Huat and WP organising secretary Ng Swee Bee were spotted entering the Elections Department building on Prinsep Street at about 1.55pm. They emerged at about 2.30pm.
Both of them, however, kept tight-lipped about who the party's candidate might be, Mr Png said: "You'll know in a few days time."
Both Mr Png and Ms Ng declined to say more.
Speculation remains about who the party might field for the Punggol East by-election, although sources say its Non-Constituency MPs Gerald Giam and Yee Jenn Jong are frontrunners.
It is understood that Mr Giam, 35, and Mr Yee, 47, have likely been ommitted from the WP's election committee, which organises and executes the party's electoral battles.
WP chief Low Thia Khiang had said on Wednesday that the committee, which usually excludes potential candidates, has been set up.