By Agence France-Presse
The employees are demanding more transparency so they can understand the moral implications of their work, said the Times, which obtained a copy of the letter.
It has been signed by 1,400 employees and is circulating on the company's internal communications system, the newspaper said, quoting three people who are familiar with the document.
The letter argues that the search engine project and Google's apparent willingness to accept China's censorship requirements "raise urgent moral and ethical issues."
"Currently we do not have the information required to make ethically-informed decisions about our work, our projects, and our employment," they say in the letter, according to the Times.
Employee anger flared with a report earlier this month in The Intercept that Google is secretly building a search engine that will filter content banned in China and thus meet Beijing's tough censorship rules.
Google withdrew its search engine from China eight years ago due to censorship and hacking.
The new project is said to be codenamed "Dragonfly."
The tech giant had already come under fire this year from thousands of employees who signed a petition against a $10-million contract with the US military, which was not renewed.
With the secret project, Google employees are reportedly worried that they might unknowingly be working on technology that could help China hide information from its people.
"We urgently need more transparency, a seat at the table, and a commitment to clear and open processes: Google employees need to know what we're building," the protest letter says, according to the Times.
At a townhall gathering of employees on Thursday, Google CEO Sundar Pichai said the firm was committed to transparency, and that while it was "exploring many options", it was "not close to launching a search product in China," the Financial Times reported, citing a person present at the meeting.