The deal announced on Monday, US time, marks a major vote of confidence for Google.
Its technology already drives much of Samsung's Galaxy AI, but the Siri deal unlocks a large market with Apple's installed base of more than two billion active devices.
"After careful evaluation, Apple determined Google's AI technology provides the most capable foundation for Apple Foundation Models," Google said, adding that its models will also power other future Apple Intelligence features.
Alphabet has been jostling with OpenAI for the Apple deal, the financial details of which were not disclosed.
The iPhone maker had in late 2024 rolled out ChatGPT into its devices, allowing the company's Siri voice assistant to tap into the chatbot's expertise to answer complicated questions.
Apple said there were no major changes to the ChatGPT integration at the time, while OpenAI did not respond to Reuters' request for comment.
"This seems like an unreasonable concentration of power for Google, given that (they) also have Android and Chrome," Tesla chief Elon Musk said in a post on social media platform X.
Musk founded his own AI firm xAI that has been trying to compete with other major players in the industry by building foundational models and spending billions on massive infrastructure.
Monday's tie-up will likely raise questions on OpenAI's partnership with Apple. In response to Gemini 3, OpenAI chief executive Sam Altman late in 2025 reportedly issued a "code red" to push teams to accelerate development.
"Apple's decision to use Google's Gemini models for Siri shifts OpenAI into a more supporting role, with ChatGPT remaining positioned for complex, opt-in queries rather than the default intelligence layer," said Parth Talsania, chief executive of Equisights Research.
Google has been firing on all cylinders to counter OpenAI's early lead in the industry by doubling down on frontier models, and image and video generation.
Apple has faced a series of setbacks on the AI front after being late to the race, with Siri's upgrade getting delayed, top-level executive changes and the initial rollout of its generative AI tools being met with lukewarm reception.
The latest agreement builds on a years-long partnership that makes Google the default search engine on Apple devices - a lucrative arrangement that drives traffic for Google while generating tens of billions in annual revenue for Apple.
News of the deal helped power Alphabet's market valuation above $US4 trillion ($A6 trillion) on Monday. The stock jumped 65 per cent in 2025 on growing investor optimism about its AI efforts.
"Apple Intelligence will continue to run on Apple devices and Private Cloud Compute, while maintaining Apple's industry-leading privacy standards," Google said on Monday, in a move to ease privacy concerns.
