Atsu has signed a five-year contract and joins the Blues for a reported £3.5million fee.
Atsu has been loaned out to Dutch side Vitesse Arnhem for the rest of the season.
Atsu made 17 appearances for Porto in the league last season.
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Since Guus Hiddink arrived at the helm, Chelsea have improved considerably after a tricky start to the campaign.
Newcastle United are staring down the barrel of relegation to the Championship for the second time in the past eight years.