Frelimo's central committee elected Chapo over three other internal candidates, in a surprise move that followed two days of intense debate, a high number of spoiled ballot papers and the withdrawal of his closest rival.
"With the election of its candidate for president, Frelimo took an important step to prepare for victory in the October elections," Nyusi said on Sunday night at the end of an extraordinary party congress near the capital Maputo.
Frelimo has won every national election since the end of the war for independence from Portugal in 1975 and also controls a large majority of local authorities.
The constitution prevents Nyusi, 65, from seeking a third term when Mozambicans vote on October 9 to choose their president, parliament and provincial authorities.
Nyusi urged Frelimo to "give Daniel Chapo all our support," after an internal election that saw Chapo's closest rival, Roque Silva, withdraw from the race at the last minute and resign from his post as Frelimo secretary general.
"We must focus on preparing the process for him," Nyusi said.
Chapo, a former political science lecturer and radio presenter, has been governor of the central province of Inhambane since 2016.
At 47, he is the first Frelimo presidential candidate to have been born after the former Portuguese colony in southern Africa gained independence.
Disputed election results
Speaking to reporters after Sunday's vote, he pledged to be a unity candidate.
"We will work... with all layers of society -- young people, women, men and war veterans," he said.
Frelimo's landslide victory in the last general election in 2019 was contested by the largest opposition party, Renamo, a former rebel group.
Renamo and Frelimo fought a civil war from 1977 to 1992 that devastated the Mozambican economy and left almost one million people dead.
In October, at least two people were killed in protests over the results of local elections, which saw police open fire on demonstrators in several cities.
Renamo called the protests after Frelimo was declared the winner in 64 of 65 local authorities.
Mozambique ranks seventh from bottom on the 191-country human development index compiled by the United Nations Development Programme.
It had high hopes for vast natural gas deposits -- the largest found south of the Sahara -- that were discovered in northern Cabo Delgado province in 2010.
But an insurgency waged by militants linked to the Islamic State group in the region has stalled progress.
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