The actor will depart in what is being billed as an "explosive, nail-biting finale" to series three, currently being filmed in Budapest.
Although Armstrong, 27, is leaving, the door will be left open for the series to continue should it be recommissioned. "Jonas leaving the show doesn't mean the end of Robin Hood," a spokeswoman told Digital Spy. "Robin Hood is more than a man - he is a hero and a legend - so how the legacy unfolds after he leaves is in itself mysterious and is guaranteed to make fans desperate to find out what happens next."
Of his decision to leave, Armstrong said: "Over the last three years I have met some wonderful people and have only amazing memories to take with me. I'm grateful to the fans for their support, but the time has come to take on new challenges and look at new opportunities."
The new series is expected to air on BBC One from January 2009.
> Click here for more news on Robin Hood series three in Tube Talk