The USL will have 273 seats in the lower Chamber of Deputies and 122 in the Senate, according to BEC data based on 100% of the ballots counted.
USL is made up of the left-wing Social Democratic Party, PSD, the National Liberal Party, PNL, and the Conservative Party, PC.
The opposition Right Romania Alliance, ARD, which comprises the centre-right Democratic Liberal Party, PDL, the Christian-Democratic National Peasants' Party and the Civic Force Party, won 56 seats in the Chamber of Deputies and 24 seats in the upper house.
The People's Party - Dan Diaconescu, PP-DD, came out third with 47 seats in the Chamber of Deputies and 21 in the Senate.
The only other party to pass the five-percent threshold to enter the lawmaking body, the Hungarian minority party UDMR, will have 18 deputies and 9 senators.
A total of 18 seats in parliament are reserved for representatives of national minorities other than the Hungarian one.
According to Romania's constitution, the president nominates a prime minister after consultations with the political formation that wins a majority in the bicameral parliament. The cabinet proposed by the prime minister-designate must then seek and win the parliament's approval.
USL leaders have said the alliance's proposal for the prime minister post is PSD head and incumbent premier, Victor Ponta.
Turnout in the Sunday election stood at nearly 41.8%. Some 18.25 million citizens were eligible to vote in the second parliamentary elections after Romania's accession to the European Union last year and the seventh since the fall of Communism in 1989.
Under changes approved before the previous parliamentary elections in 2008, Romanians no longer vote for party lists but for individual candidates.