Inter announced the transfer on their official website, with Candreva's agreement set to keep him at the San Siro until the summer of 2020.
"I am really happy and proud to be here," Candreva said in the club release. "I always wanted Inter and I'm really excited to become a Nerazzurri player at last. I'd like to thank Lazio for the seasons we spent together and for giving me the chance to realise this dream. I can't wait to play at San Siro and feel the passion of the Inter fans."
The Lazio star has spent the past four-and-a-half years at the Stadio Olimpico since joining from Udinese in January 2012 and has risen to become one of the club's most cherished assets.
Despite a low-profile start to life with Lazio, Candreva's reputation in Rome has spiked in recent times, scoring 45 goals in 192 total appearances for the club, per Transfermarkt.
Nerazzurri manager Roberto Mancini already has a talented stable of attacking talents at his disposal, with Candreva now set to duel against the likes of Ivan Perisic, Eder, Rodrigo Palacio and Jonathan Biabiany for a spot out wide.
The 29-year-old has also earned 40 caps for Italy since making his debut for the Azzurri in 2009, with 14 of those appearances coming in the past 18 months alone.
Candreva had three years remaining on his Lazio contract before agreeing his move to Inter, and he'll now look to continue his progress in Serie A after showing huge development in Italy's top flight of late.