Two state ministers and five deputy ministers were sworn-in by Sirisena at a function at the Presidential Secretariat, amid an internal bickering within the unity government.
Angajan Ramanathan, of the Sirisena-led Sri Lanka Freedom Party from Jaffna, was appointed as a deputy minister.
The main Tamil party, Tamil National Alliance, had last week objected to Mr Ramanathan being appointed as the deputy speaker of parliament.
Five out of the seven ministers appointed today are from Prime Minister Ranil Wickremesinghe-led United National Party or UNP.
The expansion is being seen as a reward to the UNP back benchers, who stood by Mr Wickremesinghe when no trust motion against the premier was brought by the united Opposition in parliament in early April.
With today's swearing-in, the number of state ministers and the number of deputy ministers has increased to 22 each.