The BBC school drama actor revealed that the English teacher will bend down on one knee to dinnerlady Maggie Croft (Melanie Hill) in tonight's series seven finale.
Speaking on Loose Women, the star teased: "I can tell you that Grantly goes down... on bended knee."
Referring to Grantly's recent struggle with his late wife Fleur's Alzheimer's, he added: "It's been great to do, a joy, but it's been pretty gritty all in all.
"Although now he's got a girlfriend so things are looking up for him after a bit of a bad spell."
Waterloo Road concludes tonight for the show's final day in Rochdale, airing at 8pm on BBC One.
Cast and crew are currently filming the next series in Scotland, the programme's new home.