Justin Bartha will guest star in the 'National Treasure' series.

The 43-year-old actor - who played sidekick Riley Poole alongside Nicolas Cage's Ben Gates in the 2004 movie and its 2007 sequel - will reprise his role in the upcoming spin-off series.

Taking to Instagram, Disney+ revealed: "Enlisting an expert 'treasure protector'.

"Let’s welcome Justin Bartha back in his guest role as Riley Poole in the #NationalTreasure series coming to #DisneyPlus!"

Bartha joins a cast including the likes of Catherine Zeta-Jones, Zuri Reed, Jordan Rodrigues, Antonio Cipriano, Jake Austin Walker and Lyndon Smith.

The series will star Lisette Alexis as Jess, described by the team as a "brilliant and resourceful DREAMer".

In the show, she "embarks on the adventure of a lifetime to uncover the truth about her family’s mysterious past and save a lost Pan-American treasure".

Marianne and Cormac Wibberley - who have co-written the pilot episode - will serve as executive producers while Rick Muirragui is on board as a writer on the series, and Mira Nair will direct.

The show is currently in production but does not yet have a release date.

Fans are hoping to see Cage back as Ben Gates in the series, but he has suggested he had fallen out of favour with Disney.

He recently said: "The phone stopped ringing. It was like, ‘What do you mean we're not doing National Treasure 3? It's been 14 years. Why not?’

"Well, Sorcerer's Apprentice didn't work, and Ghost Rider didn't really sell tickets. And Drive Angry, that just came and went."

Asked about Jerry Bruckheimer commenting on a future for the franchise, he added: "When I talk about fair-weather friends in Hollywood, I'm not talking about Jerry. I'm talking about Disney.

"They're like an ocean liner. Once they go in a certain direction, you've got to get a million tugboats to try to swivel it back around."