De Silva will succeed Lieutenant General Daya Ratnayake with effect from February 22, Sirisena's office said in a statement.
Ratnayake was in office since August, 2013 and will retire on Saturday.
De Silva becomes the 21st Commander of the Sri Lanka Army since the island gained independence from Britain in 1948.
He had joined the Sri Lanka Army as Cadet Officer in 1980 and has served in many regions including operational areas.
Sirisena has also made several changes in the military hierarchy as the Commander in Chief.
He reinstated several loyalists of Sri Lanka's only four- star general, 64-year-old Fonseka, who led the Army to defeat the Tamil Tigers in 2009 and crushed the LTTE's 30-year separatist campaign.
Fonseka later fell out with ex-president Mahinda Rajapaksa and challenged him in the election of 2010. Rajapaksa routed Fonseka in the polls. Thereafter, Fonseka was arrested, court martialled and jailed for corruption.
Fonseka loyalists also faced a purge under the previous administration of Rajapaksa.
When Sirisena defeated Rajapaksa in the bitter January presidential election, Fonseka was a leading figure in his campaign.
Soon after the polls, Sirisena pardoned and reinstated the rank of Fonseka, who was stripped of all ranks and sentenced to jail by Rajapaksa.

Latest News from World News Desk