David Cameron made his case to MPs for launching air strikes against Islamic State militants on Thursday.
He said targeting IS in Syria would make "us safer" and that the UK could not "outsource our security to allies".
But on Saturday Plaid's Leanne Wood said Mr Cameron had "failed to deliver" a comprehensive plan.
"Dropping bombs from the air will not lead to the defeat of IS," she said.
"Neither will it the secure peace for the people of Syria or bring stability to the wider region."
Welsh assembly member Ms Wood argued that UK military intervention in Syria could only be considered as part of an internationally-agreed peace plan.
"Plaid Cymru has also insisted that a framework backed fully by the UN is essential, including a Chapter VII resolution. The Prime Minister has failed to deliver this," she added.
"Unless the prime minister addresses all unanswered questions and brings forward a more comprehensive plan, Plaid Cymru cannot support military action."
Plaid Cymru has three MPs in the House of Commons.
A Commons vote authorising action against IS militants could take place next week if the prime minister gains enough support to win.
UK government ministers are expected to phone members of the opposition over the weekend in a bid to get them to support possible air strikes over Syria.
It follows the decision by Labour leader Jeremy Corbyn to write to his MPs saying he opposes action - which is supported by some members of his own front bench team.
Plaid Cymru will not support UK military action in Syria unless the prime minister can address "outstanding questions", says the party leader.