The Association of Secondary Teachers in Ireland has narrowly rejected the new public service pay agreement amid concerns over its failure to address the issue of lower pay for staff recruited since 2011.
51.5% of those who voted rejected the deal, while 48.5% were in favour.
The turnout was 64%.
The ASTI executive will now meet early next month to decide its next move - including the possible resumption of its campaign of industrial action, which was suspended during the summer pending the outcome of today's ballot.
ASTI President Ger Curtin said the union's 180 strong Central Executive Committee will now review the decision to suspend the industrial action in the wake of the ballot result.
Mr Curtin said teachers recruited since 2011 were losing out on up to €6,000 a year compared to their longer-serving colleagues, and that when multiplied by six or seven years, the loss of up to €30,000 was considerable.
He said the two-tier pay structure was creating huge difficulties in attracting teachers, with some subjects being left without qualified people to teach them.
There is no immediate prospect of disruption to schools.
Two other teacher unions - the Irish National Teachers' Organisation and the Teachers' Union of Ireland have already voted to reject the agreement, but are participating in a new process aimed at addressing the problem of pay inequality.
Mr Curtin confirmed that the ASTI will also participate in the process, adding that it was essential that all three unions be on the one page on the issue, and said he could "absolutely" envisage a concerted campaign uniting them.
He said that Minister for Finance Paschal Donohoe announcing extra teachers in the Budget was all very well, but stressed that unless the pay issue was addressed, he could "announce all he liked".