The San Jose Mercury News reports that Apple has bought the 98-acre site near its own HQ in Cupertino, with the move expected to roughly double the size of Apple's operations there.
The deal also makes Apple the city's biggest landowner.
In the summer, HP announced it was leaving the Pruneridge Avenuesite, which it had occupied for decades, and move its employees and operations to its Palo Alto HQ over the next two years. The site is said to be across the street from a 50-acre site that Apple had previously bought and announced as the location for a planned second campus.
Cupertino Mayor Kris Wang said it was great news that Apple had decided to move in to HP's old base, adding: "It's not just a company. It's the company."
Wang and other officials have said they believe the bigger Apple HQ could make up for the $1 million or so in annual tax revenue that HP contributes to the city, the News reported.
Neither Apple nor HP gave any details on the financial side of the deal, but local estate agents told the News that HP's asking price may have been around $300 million - but this could have been affected by the property market slump.
Apple spokesman Steve Dowling was reported as saying: "We now occupy 57 buildings in Cupertino and our campus is bursting at the seams. These offices will give us more space for our employees as we continue to grow."
Apple has been based in Cupertino, California, for more than three decades - Steve Jobs went to school there.
Wang and other officials said they did not know what Apple intended to do with the site, or what they had planned for the ageing buildings currently there.
HP had used the site for parts of its PC division and commercial software and hardware units. It also houses an "Executive Briefing Centre" for meetings.