David Harbour took to Instagram to let fans know how exactly he lost all that weight for "Stranger Things" season four.

In season four of the show, Harbour’s character, Jim Hopper, has been living in a Russian prison for eight months doing hard labor. The character’s situation called for Harbour to lose quite a bit of weight to make his scenes believable, and it worked, as fans were shocked when they saw him on screen.

Harbour started the post by saying he decided to address his weight loss regimen due to the number of fans who have reached out and asked how he slimmed down so much. He credited his trainer David Higgins and a strict diet as the reasons he was able to do it.

"My trainer @davidhigginslondon worked with me for 8months to make the transformation, and then another year to keep it through the pandemic," Harbour shared. "All told it was a difficult and exciting ride, changing diet and exercise plans (or lack thereof)."

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David Harbour

David Harbour worked with a trainer for 8 months to reach his goal weight, and another year after that to maintain it. (Photo by Michael Loccisano/Getty Images)

The post featured a series of pictures chronicling his weight loss journey, the first of which was a before and after split image of him in season three of "Stranger Things," and then in season four.

"All told I lost over 75lbs. 265-270 in season 3 and all the way down to 190 when we shot," Harbour explained.

After all his hard work to lose the weight, Harbour landed the role of Santa Claus for a new movie expected to come out this holiday season. He admitted in his post that he’s not having as easy a time losing the weight for season five, acknowledging the constant weight fluctuations are not good for him.

A man portraying Santa Claus sits in a mall in South Portland, Maine on Thursday, Dec. 19, 2013. Andrew Chesnut, the Bishop Walter F. Sullivan Chair in Catholic Studies at Virginia Commonwealth University, said depictions of Santa Claus as a white man came about mainly because he was a European import, a blend of the Dutch Sinterklaas and British folklore character Father Christmas, with elements of Saint Nicholas, a 4th-century Greek bishop in modern-day Turkey. (AP Photo/Robert F. Bukaty)

Harbour gained most of the weight back because he signed on to play Santa Claus in an upcoming holiday movie. (AP2013)

"I am struggling to fight back down towards a good weight for wherever hopper ends up in season 5," he said. "All this up and down is not good for the body, and I’ll have to give it up soon, but it is such a fun part of the job to live in a different version of your skin for a while."

The comments section under this post was a very complimentary place for Harbour, as celebrities and fans alike praised him for his hard work. Some commenters pointed out that the actor looked good sporting both body types and should just work towards being his healthiest self.

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Many fans also said what they want to see most is him with his "Stranger Things" co-star Winona Ryder, giving them the couple name "danona." 

These fans will be disappointed to find out Harbour is actually married… and not to Ryder, but to British singer Lily Allen. The two started dating in 2019 and got married in 2020 by an Elvis impersonator in Las Vegas, Nevada.

David Harbour and Lily Allen

Although fans would love to see Harbour and his co-star Winona Ryder get together, he is happily married to British singer Lily Allen. (Photo by Matt Winkelmeyer/MG22/Getty Images for The Met Museum/Vogue )

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The fifth and final season of "Stranger Things" is currently being written and is expected to be released sometime in 2024.