The parliamentary election will be held on November 14, the notification on Tuesday said, adding that the next parliament would convene on November 21.
The last general election in Sri Lanka was held in August 2020. MPs are elected for a five-year term.
Sri Lankans chose Dissanayake in a weekend presidential election, giving the Marxist-leaning politician a key role in deciding the future of reforms in the island country that is slowly emerging from a crushing financial crisis.
Earlier on Tuesday, he swore in opposition MP Harini Amarasuriya as his prime minister, making her the country's first woman to head the government in 24 years.
But his coalition, the National People's Party, has just three of 225 seats in the current parliament, prompting him to dissolve the legislature to seek a fresh mandate there for his policies.
Saturday's presidential vote was Sri Lanka's first election since its economy buckled in 2022 under a severe foreign exchange shortage, leaving it unable to pay for imports of essentials including fuel, medicine and cooking gas. Protests forced then-President Gotabaya Rajapaksa to flee and later resign.
Dissanayake promised to bring change for those reeling under austerity measures linked to a $US2.9 billion ($A4.2 billion) IMF bailout programme following the economic crisis and has also pledged to expand existing welfare schemes.
But his intentions to slash taxes and desire to revisit the terms of the bailout have worried investors, who fear that it could delay a crucial $US25 billion ($A36 billion) debt restructuring.
He will have to ensure the economy returns to sustainable and inclusive growth, reassure local and international markets, attract investors and help a quarter of the 22 million population climb out of poverty.
