A small but storied sliver of retail space under Rockefeller Center will get a new life on Wednesday after being shuttered for nearly a half-century.

A former Radio City Music Hall ticket booth — a 43-square-foot space in the shopping mall under the theater — is getting reopened by Happy Returns, a startup that accepts returns of unwanted merchandise purchased from online retailers.

“We are reactivating a space that was deactivated due to other modes of buying tickets,” Mark Geller, co-founder and operating chief at Happy Returns, told The Post. The original cash register was still in the booth when Happy Returns began moving in last week, he said.

The landmarked space, operated by Tishman Speyer, is Happy Returns’ first Manhattan location and one of 50 mall outposts.

The Santa Monica, Calif., company collects fees from online retailers in exchange for providing instant refunds for returns. It handles returns for 15 e-tailers, including Everlane, Eloqui, Tradesy and Shoes of Prey.

Happy Returns just raised $8 million in December — on top of a $4 million round in May — to fund an expansion to 200 locations this year.