The 21-year-old, who can play either on the left wing or as a second striker, has signed a five-year deal at Stamford Bridge.
Atsu was named Porto's young player of the season in 2010/11, and enjoyed a profitable spell on loan at Rio Ave the following year.
He was part of Porto's Primeira Liga-winning squad last season, and has made 16 appearances for Ghana, scoring three goals.
Atsu jumped the gun on his transfer on Saturday, tweeting a picture of himself holding a Chelsea shirt.
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