Jensen, known for starring in Extras, After Life, Ugly Betty and Nativity!, is set to play DI Ruth Calder, a native Shetlander who is returning home after 20 years working for the Met Police in London.
The BBC One series is based on the detective novels by Ann Cleeves and originally starred Henshall as DI Jimmy Perez, but the actor quit the series earlier this year – leaving the lead detective role vacant for Jenson's Calder.
Jensen said: "I am absolutely thrilled to be joining Shetland as DI Ruth Calder. It’s such a hugely successful show and I am aware I have very big boots to fill since the departure of DI Perez, who was very much loved by fans of the show.
"There will be a different dynamic with Ruth amongst the regular characters and a few more new interesting characters to enjoy. However the integrity of the show and the world that has been created will remain very much in the same tone as the last seven seasons.
"It's a real privilege to be asked to lead this show into a new chapter. I can’t wait to be working in Scotland again. I have missed it!"
Shetland's new series will be its eighth and will film on the Shetland Isles and across Scotland from the Spring, airing in 2023.
Fans will see DI Calder appear alongside returning characters DS Alison ‘Tosh’ McIntosh (Alison O’Donnell), Sandy (Steven Robertson), Billy (Lewis Howden) and Cora (Anne Kidd), with further casting to be announced.
Gaynor Holmes, commissioning editor for the BBC said: "I am absolutely delighted that Ashley is joining the stellar cast of Shetland.
"Ashley is an immensely accomplished and well-loved actress who is known internationally and we can’t wait to welcome her into the Shetland family when filming begins in the spring."
Henshall has previously explained his decision to leave, saying: “To be honest, I thought we’d murdered enough people on a small island."
The actor also added that the original plan had been to end the whole show, not just his character, but said that he was pleased there would continue to be more work for Scottish cast and crew.
He added: "You know, so, I just hope they don’t make a mess of it, that’s all."