Dissanayake, whose leftist coalition won 159 seats in the 225-member parliament in general election, also reappointed veteran legislator Vijitha Herath to helm the foreign affairs ministry.
Dissanayake did not name a new finance minister during Monday's swearing-in, signalling he will keep the key finance portfolio himself as he had done in September after winning the presidential election.
A political outsider in a country dominated by family parties for decades, Dissanayake comfortably won the island's presidential election in September and named Amarasuriya as prime minister while picking Herath to helm foreign affairs.
But his Marxist-leaning National People's Power coalition had just three seats in parliament, prompting him to dissolve it and seek a fresh mandate in Thursday's snap election.
The president leaned towards policy continuity as the sweeping mandate in general elections handed Dissanayake the legislative power to push through his plans to fight poverty and graft in the island nation recovering from a financial meltdown.
A nation of 22 million, Sri Lanka was crushed by a 2022 economic crisis triggered by a severe shortage of foreign currency that pushed it into a sovereign default and caused its economy to shrink by 7.3 per cent in 2022 and 2.3 per cent in 2023.
While the strong mandate will strengthen political stability in the South Asian country, some uncertainty on policy direction remains due to Dissanayake's promises to try to tweak terms of the International Monetary Fund rescue program that bailed the country out of its economic crisis, analysts said.
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