Sirisena told parliament he wanted a new statute to guarantee the country will not see a repeat of a bloody ethnic conflict that claimed over 100,000 lives between 1972 and 2009.
"We need a constitution that suits the needs of the 21st century and make sure that all communities live in harmony," Sirisena said, adding that he was ready to shed executive powers in favour of a strengthened parliament.
"The extremists in the (Sinhalese majority) south and the (minority Tamil) north have caused the loss of thousands of young lives," Sirisena said.
"We must ensure reconciliation and harmony so that we will never go back to war."
However the president -- seen as a unifying figure in the until recently conflict-torn island since taking over from strongman leader Mahinda Rajapakse last year -- acknowledged the difficulty in crafting a constitution that would satisfy both sides.
Hardline Sinhalese oppose a federal system that would ensure more political power for minority Tamils, he said.
But minorities fear that a "unitary" constitution would see them lose out to the majority Sinhalese.
Prime Minister Ranil Wickremesinghe on Saturday announced a planned "Constitutional Assembly" made up of legislators, who would seek public input and make recommendations for a new constitution.
In an interview with AFP this week, Sirisena said he was keen to abolish the island's all-powerful executive presidency and go back to a parliamentary-style democracy, which Sri Lanka had till 1978.
The new statute would be put to a nationwide referendum with hopes to complete the process early next year, he said in the interview.
Tamils who claimed they were discriminated against in education and employment took up arms in 1972 and battled against successive Sinhalese-dominated governments.
While many of the grievances were addressed over the years, the militancy grew into a full-fledged guerrilla war with Tamil Tiger rebels controlling a third of the country's territory before they were eventually crushed in May 2009.
The offensive that defeated the rebels prompted allegations of widespread war crimes, including accusations that at least 40,000 Tamil civilians were killed by government forces.
Sirisena came to power partly on the back of support from the minority Tamils after pledging reconciliation and promising investigations into war-time atrocities.
He has spoken of political power-sharing as a means to address Tamil demands for autonomy.