The science centre welcomes about 60,000 visitors a year.
Dr Matthew Evans, provost of Queen’s University, said: “This agreement reflects our joint ambition to preserve and enhance this unique site.
“The Observatory Science Centre is an important part of the Herstmonceux Castle Estate, and we look forward to working closely with Science Projects to deliver an exceptional experience for visitors.”
It follows an open letter from campaigners concerned about the observatory’s future.
Members of the Campaign Save Herstmonceux Observatory Science Centre said, while they were pleased with the new lease, it will continue its work, with a view to getting it extended to between 50 and 100 years.