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The 47-year-old actor - who plays police chief Jim Hopper in the cult Netflix series - shed 80lbs for season four in which his character held prisoner, and says his biggest weight loss tip is "being hungry".
Harbour told BBC Breakfast: "It's not easy, it's a lot of not eating when you lose that much weight, and being hungry.
"That's the secret, if you're curious about the diet secret, it's just not eating food."
But the actor - who is married to singer Lily Allen - added that he had enjoyed the challenge of changing himself for the role.
He said: "It's demanding, but it's also kind of fun. Because I like being a chameleon, I like disappearing into other lives, it's why I got into acting. So the further you can go with any of those components is great. You do it emotionally but also physically.
Watch: David Harbour fell in love with Lily Allen on third date
"So this past season I lost 80lbs. At the end of season three I was about 270lbs [20 stone] and when we came into this season I was 190lbs [13.5 stone]. It was a hard process."
He confirmed Stranger Things season five will be the last and that the show's creators had already written a "very clear ending".
He said: "I think they're pretty committed to ending it. We've talked right form the very beginning that there's series out there that over stay their welcome and you love them and then you're on season 12 and you're like 'Is that show still on?'
"The Duffer Brothers never wanted that to happen so they have a very clear ending, of which I think I know a little bit of. Next season it will end and you will know it is done."
Harbour and Allen, 37, confirmed rumours they were dating in October 2019.
They tied the knot in Las Vegas in September 2020.
Allen has two daughters two daughters — Ethel, 10, and Marnie, 9 — from her first marriage to builder Sam Cooper, which ended in 2015.