The academic leads more than 150 Fellows of the Royal Society in calling for a vote to remain.
Professor Hawking said leaving the EU would severely affect research in the UK as many young scientists were now recruited from Europe.
In the letter, he and fellow scientists wrote: “We now recruit many of our best researchers from continental Europe, including younger ones who have obtained EU grants and have chosen to move with them here. Being able to attract and fund the most talented Europeans assures the future of British science and also encourages the best scientists elsewhere to come here.”
The signatories also claim that increased funding from the EU has benefited the UK.