She joins the broadcaster after Matt Barbet stepped down last year.
Armah, who has previously worked at London Live, ITV and the BBC over a ten-year career, said: “I’ve long admired 5 News and how it puts people at the heart of the story.
“I’m incredibly excited to be joining the talented and creative team, particularly as we chart the largely unknown political landscape for our viewers.”
Sian Williams continues to front the 5pm bulletin.