The Welsh nationalists had backed the Remain campaign, with party officials fearing Brexit would have devastating consequences for Wales when billions of pounds of EU funding programmes stop.
Plaid’s National Executive Committee met in Rhayader, mid Wales, on Saturday, to discuss the vote and its aftermath.
Following the meeting, party chairman Alun Ffred Jones said: “The Leave campaign made promises during the course of the referendum campaign that EU funding schemes would be replaced pound for pound by the UK government.
“And promises were made that extra money would come to Wales to spend on the health service.
“It looks increasingly likely that these promises will now be broken.
“Plaid Cymru will fight tooth and nail to hold Leave campaigners to account over their vow to people in Wales.
“We will leave no stone unturned in securing Wales’ future.”
The EU referendum saw 53% of Wales vote to leave the EU - which contrasted sharply with Celtic cousins in Scotland and Northern Ireland, who both chose to remain.
And in Blaenau Gwent in south Wales, where European money was put towards a £350 million regeneration project, 62% of people wanted out.
But Plaid leader Leanne Wood, who was born and raised in the Rhondda Valleys, previously said she was not surprised by the result.
The former probation officer described the referendum as a chance to “land a blow on the political establishment”.
Plaid have since gone on to claim that the issue of Welsh independence should be on the agenda following the referendum.
On Saturday, hundreds of people attended rallies in Cardiff and Caernarfon calling for Wales to leave the UK.
However, Plaid’s ultimate goal of independence could be difficult to achieve.
Coupled with the large Leave vote in Wales, Plaid do not have the same clout as the SNP in Scotland and have never formed their own government in the Welsh Assembly.
And recent polls put the support for Welsh independence at only 17 per cent.
Last week, Conservative MP and former Secretary of State for Wales David Jones called Plaid’s Ms Wood “out of step” with voters.
First Minister and Welsh Labour leader Carwyn Jones brushed off talk of Wales going it alone, but suggested the referendum could lead to a “federal UK”.
He told BBC’s Welsh language news programme Newyddion 9: “The last thing we want is to be considered some sort of annexe to England.
“We don’t have to have independence - there is a way to ensure there is some sort of federation, or some sort of agreement.”