The 42-year-old, previously at Spanish club Valencia, was the fourth coach to be appointment by Porto since they won their last major trophy, the league title in 2013.
Usually known simply as Nuno, the Sao Tome and Principe-born coach replaces Jose Peseiro, who was dismissed on Monday.
"This is not the moment for promises but for guarantees," Nuno said in a Porto statement.
"I am a person who follows his convictions and my over-riding conviction is that we can always win. I guarantee to the nation of Porto supporters that, with hard work and unity, we will achieve what we all want, which is to win."
Nuno's playing career took him to Vitoria Guimaraes, Deportivo La Coruna, Merida, Osasuna, Dynamo Moscow and Aves, although he was often second-choice.
He had two stints at Porto, the first from 2002-04 when he played second fiddle to Vitor Baia and the second from 2007-10, when Brazilian Helton stood in his way.
Nevertheless, he won one Champions League, one Intercontinental Cup, one UEFA Cup, four Portuguese league and three Portuguese Cup winners' medals during those two spells.
After starting his coaching career as assistant to Jesualdo Ferreira at Malaga, he was named Valencia coach in July 2014 but was fired one year and four months later for what the club described as "less positive" results.
Porto won the Portuguese league nine times out of eleven between the 2002 and 2013 but their dominance came to an abrupt halt and arch-rivals Benfica have won the league for the last three years.
Since 2013, they have appointed Paulo Fonseca, Julen Lopetegui and Peseiro as coaches, while Luis Castro and Rui Barros have also been in charge on an interim basis.
(Writing by Brian Homewood in Berne; Editing by John O'Brien)
