Atusu has joined the Blues on a five-year-contract for a reported £3.5m fee and will be loaned out to Vitesse Arnhem for the rest of the season.
The Ghana international is a left-sided player who can play as a winger or second striker.
Atsu joined Porto in January 2010 from Cheetah FC and finished the season with the club's Youth Player of the Season award.
Andre Villas-Boas included him in Porto's senior side for the first time in the same season but he remained on the bench.
Atsu spent the 2011/12 season on loan with Rio Ave where he made 30 appearances and scored six goals, earning him the Player of the Season gong.
The 21-year-old broke into Porto's first team last term, turning out 17 times in the league and also played in a Champions League last-16 tie with Malaga.
Ghana called Atsu into their senior side in 2012 and he has been a regular since, scoring in the Africa Cup of Nations earlier this year.