The Foreign Ministry said the Central American nation, which has no oil assets of its own, joined after Petrocaribe member-state energy officials agreed and gave the green light.
The accession papers were signed Monday by President Salvador Sanchez Ceren, a former leftist guerrilla, who took office Sunday.
Through Petrocaribe, OPEC-member Venezuela sells about 100,000 barrels of oil a year worth about four billion dollars; it takes payment of half that in cash, and the rest in goods and services, mainly food.
Petrocaribe groups Antigua and Barbuda, the Bahamas, Belize, Cuba, Dominica, the Dominican Republic, El Salvador, Grenada, Guatemala, Guyana, Haiti, Honduras, Jamaica, Nicaragua, St Kitts and Nevis, St Lucia, St Vincent and the Grenadines, Suriname and Venezuela.