The victory comes after Rajapaksa was re-elected in the country's presidential poll in January.
His party's closest rival, the United National Party (UNP), won 60 seats in total, results showed.
Another opposition party led by Sarath Fonseka, the defeated presidential candidate and former army chief, received seven seats, with Fonseka himself one of those to be elected.
The UPFA had already secured 117 seats in the April 8 election, allowing the party to form a government with a simple majority.
But it fell six seats short of its goal to reach a two- thirds majority in order to make constitutional changes.
Though Rajapaksa has not specified the proposed changes to the constitution, opponents have speculated that they may include electoral changes.
The constitution currently prevents him for standing for re-election when his second term ends in 2016.
Sri Lanka's opposition has been weakened and fragmented after the arrest of Fonseka, who split with the president after the government's victory over the separatist Tamil Tigers in the country's north.
Held in military custody on charges of sedition, Fonseka has run his campaign for a parliamentary seat while in detention.
The opposition accuses Rajapaksa of stifling dissent, encouraging cronyism and corruption and trying to establish up a family dynasty, with two of his brothers and a son running for parliament and other relatives occupying senior government posts.