Josh Hawley said his letter followed a meeting that Google executives had held with US President Donald Trump to discuss the company's stance on working with the Defence Department.
Google came under pressure from its staff after it signed a deal with the Pentagon to work on a project, known as Project Maven, that provides the military with AI technology to better identify images on the battlefield.
Staff at Google had expressed disquiet over the company's involvement in the project. More than 3000 staff signed a letter submitted to senior management to protest against the decision to get involved in Maven.
Following the protests, Google said it would pull out of Maven by the end of the year but gave no assurance that it would not take up similar work in the future.
Hawley's letter came after he had spoken to acting defence secretary Patrick Shanahan and chairman of the Joint Chiefs of Staff, Joe Dunford, and the two officials had told him that Google's work in China was benefitting the Chinese military.
Hawley wrote: "I have been deeply troubled by the testimony of our national defence leadership regarding Google’s unwillingness to partner with the US Armed Forces, even while pursuing lucrative projects in China that directly or indirectly benefit the repressive Chinese Government."
In August last year, it was revealed that Google was planning to re-enter the Chinese search market with a censored search utility. After this was revealed, Google decided later in the year to put the project, known as Project DragonFly, on hold.
"According to reports, the work Google is doing with China not only includes development of artificial intelligence technology that may possibly be used by Chinese military and intelligence services to exploit Americans’ data and privacy, but also the introduction of platforms that advance a value system and modes of behaviour fundamentally at odds with our own, such as the censor-friendly search engine Project Dragonfly," Hawley said.
"That Google persists in seeking partnerships in China – including building an artificial intelligence facility in Beijing – suggests a long-term commitment by the company to have China as a partner."
Hawley urged Pichai to publicly speak about the work Google was doing in China and the company's reluctance to partner with US Armed Forces.