Sylvain Rochon took the riding of Richelieu, which has voted PQ since 1994, and became vacant last fall when Elaine Zakaib stepped down.
Rochon, who won with 36 per cent of the vote, was a former adviser to Zakaib.
The Liberals have 69 seats in the 125-member national assembly, compared with 30 for the PQ.
The Coalition for Quebec's Future has 22 seats, Quebec solidaire has three and there is one vacancy.
The Richelieu riding is northeast of Montreal.