Alvarado, from the ruling center-left Citizens' Action Party, is promising to more vigorously pursue the policies of outgoing President Luis Guillermo Solis while avoiding the influence-peddling scandals that gripped Solis's government.
Late Sunday, in his post-election victory speech, he emphasized national "unity" as he vowed to boost education and infrastructure spending, while bringing down the country's ballooning deficit.
His Citizens' Action Party holds just 10 seats in the 57-seat Legislative Assembly, so will need to build a coalition to rule.
He sent letters to the seven groupings represented in the new congress, inviting them to take part in in his government. The congress will sit from May 1 following legislative elections in February.
The president-elect told them he had received a "clear mandate" to form "a national-unity government bringing together different political forces."
Alvarado, a 38-year-old former labor minister, writer and onetime journalist, in Sunday's run-off election beat a right-wing evangelical preacher, Fabricio Alvarado (no relation), for the job as head of state.
Carlos Alvarado won 61 percent of the vote, against 39 percent for Fabricio Alvarado, who had surged from obscurity in the February first round by harshly criticizing moves to recognize gay marriage.
Some of the priorities facing him are a deficit that has grown to 6.2 percent of gross domestic product, unemployment of more than nine percent, and a serious shortfall in past infrastructure investment.
"Costa Rica needs to work and resolve many problems," he said in his victory speech.
Carlos Alvarado will take power next month, for a four-year term in the relatively prosperous Central American country whose region is beset by poverty and violent crime.
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