Tom Quinn, the protagonist in the first two series, was last seen early in the third series when the MI5 agent was decommissioned for almost sabotaging an operation involving a sleeper agent.
Macfadyen, however, is to appear in the last episode of the spy drama, though details surrounding his return are being kept under wraps.
An insider told The Mirror: "It's incredibly exciting to have Matthew back again, even if he's only seen fleetingly. It will bring back all the memories of Tom and what the character endured in those first few series.
"Viewers will be dying to know why he's back, but they'll have to wait until Sunday to find out."
Spooks stars Lara Pulver and Geoffrey Streatfeild previously gave hints about the show's end, teasing that "they're not all going to make it".
Watch Pulver and Streatfeild talk about the finale to Digital Spy below: