The DR Congo international signs for the Primeira Liga side for an undisclosed fee, reportedly in the £5.5million region.
In his three years on Tyneside the 23-year-old made a total of 59 appearances.
His only goal in that time came on the final day of the Championship promotion season, as United secured the second tier title with a 3-0 win over Barnsley in May 2017.
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Mbemba, who joined from Anderlecht in July 2015, could make his Porto debut this weekend against his former side when Rafa Benitez's men travel to Estádio do Dragão for their third pre-season game of the summer to date.
Porto entertain Benitez's Magpies on Saturday.
Mbemba's exit could see United up their interest in defender Deportivo La Coruña defender Fabian Schar.
The Mail have reported the 26-year-old could be set for a move to Tyneside after the Spanish outfit's relegation from the top flight last season.
A move for Schar, who represented Switzerland at the World Cup, is said to have been 'discussed', and the centre back is unlikely to command a large transfer fee.
Meanwhile, Newcastle have also been credited with an interest in Bristol City full-back Joe Bryan.
Reports in the Sun claim that Benitez is keen on the £6m-rated full-back, who has also been linked with Middlesbrough and Leeds this summer.
With just one year remaining on his current contract at Ashton Gate, Bryan is being highly-tipped to leave the Robins before the transfer window slams shut.