The Russian giants also managed to secure the services Axel Witsel for a reported £30million from Benfica on a five-year deal.
Hulk is believed to be a long-time target of Chelsea but the Russian giants have ended speculation linking him with a Stamford Bridge move after confirming their swoop for the forward on their official website on Monday evening.
The club did not disclose the transfer fee but reports claim they have parted with €50million (£39.6m) to secure the frontman.
The 26-year-old made a huge impact at Porto after joining from Tokyo Verdy in 2008.
But the Brazil international had long been linked with a new challenge, with his agent admitting earlier in the summer that the "time was right" for the player to move on.
Last week, Porto president Pinto da Costa revealed he had turned down an offer worth €50m from Zenit for the powerful forward, but a deal has now been reached by the two clubs.
Hulk scored 21 goals in all competitions last season and played a crucial role in Porto's Europa League title-winning campaign in 2010-11.
Belgian international Witsel, 23, only joined the Portuguese side in 2011 from his hometown club Standard Liege, where he started his career in 2006.
He will link up with fellow new signing Hulk as the Russian champions prepare for the Champions League this season in Group C against AC Milan, Anderlecht and Malaga.