Analysts had expected the NPP to do well in the elections but what remained to be determined was the margin of victory, and whether it would get the two-thirds majority it wanted to be able to pass its ambitious reforms.
In the outgoing assembly, Dissanayake's Janatha Vimukthi Peramuna (JVP) party, which now leads the NPP, had just three seats.
The 55-year-old earlier told reporters that he believed this was "a crucial election that will mark a turning point in Sri Lanka".
Sajith Premadasa, the man Dissanayake defeated in the presidential elections, led the opposition alliance.
Dissanayake called for snap elections shortly after he became president to seek a fresh mandate to pursue his policies. There was "no point continuing with a parliament that is not in line with what the people want", he had said.
Nearly two-thirds of former MPs had chosen not to run for re-election, including prominent members of the former ruling Rajapaksa dynasty.
Out of the 225 seats in the parliament, 196 MPs were directly elected. The rest were nominated by parties based on the percentage of votes they get in what is known as proportional representation.
