The oil-rich, cash-poor South American country failed to make the payments on bonds due 2025 and 2026, S&P said in a statement.
The ratings agency also warned of a "one-in-two chance that Venezuela could default again within the next three months."
"Two additional coupon payments are overdue, but within their grace period. We could lower the ratings on the following issues to 'D' if the government fails to pay within the stated grace period," it said.
Struggling under an estimated debt burden of USD 150 billion, Venezuela's President Nicolas Maduro said earlier this month that he wants to restructure Venezuela's sovereign debt as well as that of PDVSA.
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PDVSA is the primary source of income for Venezuela, a country that sits atop the world's biggest oil reserves.
S&P said it would "very likely" consider any Venezuelan restructuring equivalent to a default.
"In addition, in our opinion, US sanctions on Venezuela and government members will most likely result in a long and difficult negotiation with bondholders," it said.