Google has gained tremendous faith in US patient data ̵
The scheme, called Project Nightingale, was agreed with Ascension, which operates 2,600 hospitals.
Google can access health records, names and addresses without telling patients, according to the Wall Street Journal, which first reported the news.
The tech giant said this was "standard practice".
Among the data that Google allegedly has access to under the agreement are laboratory results, diagnoses, records and date of birth.
Neither doctors nor patients need to be told that Google can see this information.
The Wall Street Journal reports that data access began last year and was expanded through the summer.
In a blog, Google said that Ascension's work would comply with industry regulations such as the US Health Insurance Portability and Accountability Act of 1996 (HIPAA).
"To be clear … patient data cannot and will not be combined with Google consumer data," the company added.
Ascension said the agreement will help it "optimize" patient care and will include the development of artificial intelligence (AI) tools to support doctors.
The company also said it would start using Google's cloud storage service and business applications known as G Suite.
However, Project Nightingale has already attracted criticism from those who claim it removes patients' control over their own data.
"It is a huge question that these public-private partnerships are all made under private contracts, so it is quite difficult to get some transparency," said Prof Jane Kaye of the University of Oxford.
"Google says that they do not link them to other data, but what they do all the time is refine their algorithms, refine what they do and give them [selves] the market advantage."
Health organizations are under increasing pressure to improve the efficiency and quality of care. Many people turn to AI in an effort to sharpen services, but such moves have sometimes faced criticism over how sensitive patient data is handled.
In the United Kingdom, Google's AI-focused subsidiary DeepMind was found to have broken the law when it failed to properly explain to patients how their data would be used in developing a kidney disease app.
The tool, called Streams, was designed to flag patients at risk of developing acute kidney injury.