Sirisena expanded his Cabinet to 40 by handing out ministerial posts to 11 opposition legislators while five others were made ministers of state and 10 deputy ministers.
It was the biggest induction of opposition lawmakers into any Sir Lankan government since independence from Britain in 1948.
They will join the government headed by SLFP's arch-rival United National Party leader Ranil Wickremesinghe, who is the Prime Minister.
Analysts say Sirisena has handpicked those who have been treated less favourably by defeated former long-time president Rajapaksa.
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Minster and Cabinet Spokesman Rajitha Senaratne said that the Government has decided to form a coalition with the SLFP.
"UNP-SLFP national government. This will be a historic decision," the Minister said.
Soon after winning the January 8 presidential election, Sirisena appointed the then-opposition UNP leader Wickremesinghe as prime minister heading a minority government.
Despite the UNP having only 43 seats in the 225-member Parliament, Sirisena made Wickremesinghe the Prime Minister in keeping with the election pledge to do so.
The Sirisena-Wickremesinghe government has pledged a fresh parliamentary election after implementing its constitutional and electoral reforms aimed at greater democracy.