Members of the Action democratique du Quebec finalized the merger with the upstart Coalition, which promises to set aside the debate on sovereignty for a generation.
The merger means the Coalition gets the ADQ's four seats in the legislature and inherits its research budget.
The party, led by former Parti Quebecois cabinet minister Francois Legault, now holds nine seats despite not having taken part in an election.
Two former ADQ members and another two ex-PQ members had already joined the Coalition.
Party leaders and their executives agreed to the merger in December, but ADQ members had the final say in a mail-in ballot.
More than 70 per cent of those who voted approved the deal, though only 54 per cent of ADQ members cast a ballot.
ADQ leader Gerard Deltell said the merger would be good for democracy in Quebec and that his party's goals would live on with the Coalition.
"Now we will continue our efforts with the Coalition for Quebec's Future to provide the nation with a realistic and responsible government," he told reporters Sunday.
"With our values and beliefs... we will present Quebecers with a concrete alternative to the Liberals."
The Coalition has been leading the ruling Liberals and the Opposition Parti Quebecois in public opinion polls for several months, even before it became an official party.
The party was originally billed as right wing but shifted more to the centre in developing its platform.
The ADQ took more conservative positions over the years, such as endorsing private health care and school vouchers.
It had been around for two decades, briefly gaining Official Opposition status in 2007 before dropping off in recent years.
ADQ membership nosedived to 2,521 from the 50,000 it held at the height of its popularity in 2002-2003.
The merger means the Coalition will be able to use the ADQ's research and operations budget of over $700,000.
Quebec Premier Jean Charest is in the fourth year of a five-year mandate. He doesn't have to call an election until December 2013.
Note to readers: This is a corrected story. A previous version carried an incorrect spelling of Gerard Deltell's first name.