Ninety per cent of eligible flight attendants voted on the agreement, with 57 per cent of those voting casting ballots against the proposed five-year deal, the airline said.
The tentative agreement was reached between the airline and the WestJet Flight Attendant Association Board. WestJet's flight attendants are not unionized, but represented by an employee association.
The airline says it will meet in the coming weeks with the association board and focus on understanding specific concerns, following the steps outlined in a dispute resolution process.
The Flight Attendant Association Board is part of the ProActive Communication Team employee association which represents some 9,500 non-management employees at the airline.
Anthony Pascale, chairman of the Flight Attendant Association Board, said he was disappointed by the vote.
"We will survey our members in the coming weeks and re-engage them in the new year," Pascale said.