Congress swore in Moreno, 64, as the quieter successor to one of the feistiest personalities in Latin American politics: Rafael Correa.
Moreno is the first wheelchair-user to become Ecuador's leader, and one of few such leaders in the world ever to serve as president.
His legs have been paralyzed since he was shot during a robbery in 1998.
He went on to lead a task force on disability rights as vice-president in Correa's government. That earned him a nomination for a Nobel Peace Prize in 2012.
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He won the hearts of many with welfare spending, social equality policies and subsidies which he says reduced poverty.
And unlike his allies in Brazil, Argentina and Peru, his side has managed to stay in office, winning re-election this year.
Known to his supporters simply as "Lenin," Moreno "is willing to be less confrontational and to have a softer approach as president to the opposition and the media," said Farith Simon, an analyst at San Francisco University in Quito.
Moreno said in his program for government that "passion for life obliges us to deepen the changes we have achieved and defend our social progress."
Economists warn Moreno faces tougher conditions than those enjoyed by Correa, however.
Like other Latin American countries, Ecuador has suffered from falling prices for its oil and minerals.
"There is an economic hangover," said Simon.
The economy soared after Correa took over but fell back last year, shrinking 1.5 per cent.
Ecuador's external debt has climbed to more than USD 25 billion -- over a quarter of its output.
Correa's time in office has been marked by his abrasive personality. He vocally criticized his opponents and the media, which he branded as "corrupt" and "lying."
Ecuador had been dubbed ungovernable when he came to office in 2007, with seven different presidents over the preceding decade.
He launched vigorous reforms, boosting social spending, curbed oil firms' profits and suspended some debt payments that he considered illegitimate.
"We succeeded. I am handing over a country totally different from the one I received," he said recently.
More conciliatory in style, he has pledged to boost business through loans and try to spur consumption.
His cabinet, announced yesterday, includes business- friendly figures in key economic roles as well as social leaders.
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