Meadow Brown Restaurants has won a seven-year contract worth £7.5m with Nottingham Castle Trust.
The castle is going through a £29.4m regeneration to redevelop the site, including the ducal palace, grounds and the sprawling cave systems hidden within the castle rock, and is planned to reopen on 29 March 2021.
Meadow Brown will operate the newly added visitor centre café next to the play park, as well as the ducal palace restaurant within the castle itself. The group said it would be investing "heavily" in events at the castle.
"We are extremely excited to be the part of this exciting project and cannot wait to get started," said managing director Raj Regmi.
"We have always wanted to operate a visitor attraction venue and we could not have thought of a better place than Nottingham Castle steeped in history and the famous tale of Robin Hood, of course. What an amazing venue!"
The family-owned business, run by husband and wife Raj and Neera Regmi, also operates the Ferry Meadows café and Lakeside Kitchen & Bar at Nene Park in Peterborough, and the Woodland Manor hotel in Bedfordshire.