The 25-year-old, who passed a medical with the ambitious Serie A club last week, has signed a reported five-year contract at the Stadio San Paolo with reports in Italy suggesting Real have received 40million euros (£34.5million) for the forward.
A statement published on Napoli's official website read: "Napoli have made the signing of Gonzalo Higuain official. The Argentinian striker arrives from Real Madrid."
Higuain, who scored 107 league goals in six seasons with Real Madrid, becomes the latest addition to Rafael Benitez's Napoli squad, with the former Liverpool boss now focusing on ratifying a season-long loan deal for Reds goalkeeper Jose Reina.
Reina passed a medical at Rome's Villa Stuart sport clinic in the hours that followed Higuain's arrival to a rapturous reception at Fiumicino airport on Wednesday.
Napoli president Aurelio De Laurentiis has promised "a big party" to present the Argentinian ahead of a friendly match against Galatasaray on Monday night.
Benitez knows Reina well, having brought the Spain international to Anfield from Villarreal in 2005, but he is particularly excited to work with Higuain.
Benitez told the Corriere dello Sport last week: "We were looking for a quality player. We were looking for someone playing at the highest level.
"We have signed a player who comes from Real Madrid and we have signed a striker who has scored a hatful of goals and who will adapt very well to our system - an attacking system. We needed a player like this.
"We are all working together to ensure the players we want arrive. We have to sign players who will do well for Napoli for the next three or four years."
Benitez, who succeeded Walter Mazzarri at the San Paolo in June, has already signed winger Dries Mertens from PSV Eindhoven and both forward Jose Callejon and defender Raul Albiol from Real Madrid.
Last year's Serie A runners-up are rebuilding for both a title challenge and a Champions League campaign, and the recent flurry of transfer activity has been sparked by the arrival of the 64million euros (£55million) earned from striker Edinson Cavani's transfer to Paris St Germain.
Until signing off on a move to Naples, Higuain had been a reported target for Premier League side Arsenal and he is now likely to face the Gunners in a Napoli shirt at the Emirates Cup.
The pre-season event, which also involves Porto and Galatasaray, is being hosted Arsenal on the weekend of August 3-4.