They said the project raised “urgent moral and ethical questions” and urged the firm to be more transparent.
“Currently we do not have the information required to make ethically-informed decisions about our work,” they added.
Google, which has never spoken publicly about the plans, declined to comment.
The firm, which is owned by Alphabet, quit China eight years ago in protest at the country’s censorship laws and alleged government hacks.
However, reports last month claimed it had been secretively working on a new Chinese search service, referred to internally as Dragonfly.
The platform, which still requires Chinese government approval, would block certain websites and search terms like human rights and religion.
This has angered some employees who fear they have been unwittingly working on technology that will help China suppress free expression.
In their letter, which was shared with various media organisations, they also argue it would violate the “don’t be evil” clause in Google’s code of conduct.
“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 letter said.
It is not the first time Google employees have spoken out against the company’s decisions.
In April, thousands of staff criticised its work on a US military programme developing artificial intelligence for drones.
Google has since ended its AI contract with the Pentagon.
China has the world’s largest internet audience but US tech firms have struggled to take off in China due to content restrictions and blockages.
Facebook, Google, Twitter and Instagram are all banned, although Google still has three offices in the country.
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