The 53-year-old Scotsman is a machine learning expert who joined Google in 2010 when the search giant acquired a startup called Metaweb, where he was chief technology officer. Giannandrea will be one of 16 Apple executives to report directly to the company's chief executive, Tim Cook.
According to the New York Times, which first reported the hiring, Giannandrea will head up Apple's "machine learning and AI strategy". This will likely by the strategy Apple uses to get itself back into the AI game, having led with the launch of Siri back in 2011 but since found itself overtaken by rivals Amazon and Google with Alexa and Google Assistant respectively.
It is also expected that Apple will use Giannandrea's experience to improve its own facial recognition software for correctly tagging users' photos, and for turning Siri into a more able and intelligent smart home assistant - an area where Apple lags behind the competition.
The iPhone maker will seek to do this while retaining a high standard of privacy for its users.
Cook broke the news to Apple staff via an email, stating: "Our technology must be infused with the values we all hold dear. John shares our commitment to privacy and our thoughtful approach as we make computers even smarter and more personal."
Giannandrea's experience in developing Google's internet search, Gmail and Google Assistant products puts him in the ideal place to help improve Apple's AI offerings across its products and services.
As Alexa and Google Assistant hog the limelight with their intelligence, Apple finds itself on the back foot - especially after launching its HomePod smart speaker with a version of Siri inferior to that on iPhones and Macs.
A recent experiment found Siri on the HomePod could only answer 52.3 percent of 782 questions correctly. Despite understanding 99.4 percent of what was asked, Siri couldn't act on the instructions given, or provide answers to questions asked.
Google Assistant answered 81 percent of queries correctly, while Amazon Alexa scored 64 percent and Microsoft Cortana got 57 percent.
Giannandrea will be replaced by Google veteran Jeff Dean, who founded Google Brain and is now in charge of the company's entire AI division.