The Move Forward Party — which has called for an overhaul of the military and amending a strict law that criminalizes criticism of the Thai monarchy — secured 151 seats out of the 500-member House of Representatives. The result defied opinion polls, which had predicted a strong victory for Pheu Thai, the country’s largest opposition party, founded by former Prime Minister Thaksin Shinawatra.

Pheu Thai won 141 seats, which, like Move Forward, was short of a clear majority. The two parties announced during separate news conferences on Monday that they had agreed to work together to form a government.

Mr. Pita has led the effort to build the coalition. He said that five parties, including Pheu Thai, had already joined him, boosting the opposition’s control over Parliament to 309 out of 500 seats. “It’s safe to assume that we have secured a majority in forming a government,” Mr. Pita said on Monday.

Prime Minister Prayuth Chan-ocha, the general who seized power in the 2014 coup, said on Sunday that he “has respect for the democratic process and the election results.” His party, United Thai Nation, won only 36 seats.