Rajapaksa's ruling United Peoples Freedom Alliance (UPFA) secured 144 out of 225 seats in the parliament after re-polling on Tuesday in two areas hit by violence at the April 8 legislative election.
D M Jayaratne, a former teacher and postmaster who joined in politics in 1951 as one of the earliest members of Rajapaksa's Sri Lanka Freedom Party, took his oath as prime minister.
With Rajapaksa having the parliamentary majority and strong executive powers, Jayaratne will have a largely ceremonial role although will be the leader of government business in parliament.
At Sri Lanka's first parliamentary vote since defeating the Tamil Tiger rebels 11 months ago, Rajapaksa came only six seats shy of the two-thirds majority he needs to change the constitution.
However he is expected to work on getting opposition members to cross over to secure it and push through reforms.
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