Plant's presence at the Folk at the Castle night, part of two days of music at Ludlow Castle which capped off the three-week arts festival, was kept strictly under wraps right up to the day of the show.
The former frontman of 70s rock superstars Led Zeppelin performed four traditional folk songs with Shropshire area musicians Grey Wolf (Jim Allen, Martin Thomas and Ben Walsh) and singer Deborah Rose.
His appearance was only officially announced on the festival's Twitter feed on Saturday, hours ahead of the gig.
Festival director Anita Bigsby said: "I'd known for a week that he might be performing and wanted to, and was rehearsing with Grey Wolf. But he didn't want any publicity for it at all. It was only on the the day we got the go ahead to start shouting it out."
She said the following night at the castle, which saw a mix of vintage swing and modern electro beats with the Electro Swing Circus also went down a storm.
"It was really vibrant and upbeat," she said.
Robert Plant's performance follows another surprise appearance at Tenbury Music Festival two weeks ago.