Lawmaker Kingsley Wong has tested positive for Covid-19.
Wong, who's the chairman of the Federation of Trade Unions, said on Thursday that he is self-isolating.
He said he had received four doses of Covid vaccine.
The lawmaker said he had mild cold-like symptoms and would take some Chinese medicine.
On Tuesday, lawmaker Chan Siu-hung and an interpreter who works at Legco tested positive for coronavirus.
Legco said it had arranged disinfection of the complex.