Lord - who played teen Jay Brown's father Jase Dyer for a year until he was killed off in 2008 - takes on the role of Kirsty Clements's (Lucy Gaskell) husband Warren.
A dark and brooding character, Warren's arrival leads to viewers discovering a little more about Kirsty and her family.
Series producer Oliver Kent told DS: "We're really excited that Stephen Lord is joining Casualty as Kirsty's husband. He's a really good actor with a great range and we're going to see some really powerful stuff from him and Lucy Gaskell when we delve more closely into Kirsty's home life and marriage."
Lord is due to begin filming at the long-running drama's home in Bristol in a few weeks and will first appear a short way into the new series.
Last week, DS exclusively revealed that former EastEnders' star Michelle Collins - Cindy Beale in the Walford drama - is to film a multi-episode guest stint for the next series of Casualty.
Series 25 is due to begin transmitting from September.
> Click here to read our recent interview with Oliver Kent