Kingston's Frère Jacques has sponsored the Rose for £50,000, making the riverside restaurant the theatre's biggest benefactor outside Kingston Council and Kingston University.
Restaurant owners John and Clare Scott came forward with the money just two weeks after the Comet and theatre trust launched campaigns to find £600,000 in sponsorship.
Other companies and individuals have expressed an interest in helping the Rose, so the figure is expected to rise dramatically in the next couple of weeks.
Mr Scott, 66, who has run Frère Jacques for 13 years, said: "This theatre is going to be the bee's knees.
"Shakespeare would have given his right hand, well, his left hand, for a theatre like this. I think it is going to be absolutely wonderful.
"It is something I am very pleased to be involved with. As a company we have been supported by the people in Kingston for the last 13 years and have done very well out of them.
"We have always tried to help Kingston University when it puts on exhibitions but this is on a much grander scale for us. It is actually a bit of a stretch, but we wanted to give something back.
"When we see the first solid Shakespeare production in there the hairs will stand on the back of my neck."
The restaurant, as well as the huge financial donation, has agreed to host fundraising evenings and entertainment.
As a corporate sponsor, the restaurant will get marketing opportunities, tickets and access to the venue for special events.
David Fletcher, executive director of the Rose, said: "Earlier this month, the Rose announced a £600,000 fundraising target and has been working with the Surrey Comet to raise the profile of its campaign.
"Frère Jacques' generous sponsorship has signalled a hugely positive move from the local business community towards achieving this target. We look forward to working with Clare and John Scott and the team at Frère Jacques."
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