Voronin won 75 votes in the assembly, 14 more than the 61 required to secure election.
The communists were forced to put forward two candidates after parliament's two opposition groups, the Christian Democrat Popular Party (CDPP) and the Democratic Moldova Bloc (BDM), declined to nominate anyone.
The former did not have enough deputies to field its own candidate, and parties that make up the BDM failed to agree on a single candidate.
Moldovan law dictates that there must be two candidates if a presidential election is to be valid.
The president needed to get 61 votes to be elected, meaning that Voronin had to get at least five votes from a party other than the communists, though observers expected him to easily garner the required support.
The two main opposition parties
declined to nominate candidates
"First of all, it's not in the interest of the deputies to have another round of voting," Arcadie Barbarosie, an analyst in the capital Chisinau, said.
"Secondly the parties know that it wouldn't be good for the country. The country is now partially at a deadlock until a president is elected."
Voronin has been in power since 2001 when the communists were elected in a landslide victory on a pro-Russian ticket during parliamentary elections.
During the past several years, however, Voronin and his party have found themselves increasingly at odds with Moscow, most of all over the Transdniestr region, Moldova's pro-Russia separatist republic that stretches along much of its eastern border with Ukraine.
Opposition's Serafim Urechean
failed to unseat ruling leftists
During the March parliamentary elections, the communists campaigned on a pro-Western ticket, including eventual membership in the European Union.
Moldova, sandwiched between Romania and Ukraine, is considered to be Europe's poorest nation.
A Romanian-speaking province that imperial Russia and the Ottoman Empire fought over for centuries, modern-day Moldova was annexed by the Soviet Union at the end of the second world war.