Harris's spot in Australia's Ashes team has been in the spotlight since his latest poor return in Adelaide, making for 14 dismissals since he last reached 50.
Figures this week showed the his Test average of 22.19 is the lowest of all regular Australian openers in 128 years, headed into the third Test.
But just as the likes of Steve Waugh and others did when Langer's spot was under pressure during his own playing career, Australia's coach will stand by his opener.
"He'll play in the Test, no worries about that," Langer said on Thursday.
"This is his home ground. He's played a lot at the MCG.
"He hasn't made the runs he'd like to so far, but he dominates domestic cricket.
"So he knows that he knows how to play.
"He's a fantastic bloke around the squad ... And we know he's a very good player.
"For him and for us we're hoping he plays well and gets a good partnership with Davey Warner in this Boxing Day Test match."
Langer's comments come after Steve Smith and Travis Head also came out in support of the Harris — after he made scores of three and 23 in Adelaide.
Chief selector George Bailey also made clear weeks before the series Harris was the man to open with Warner, and he would have a long run at the role.
Langer shares a close bond with Harris, having known him since his early teens as a fellow left-handed opener coming through the West Australian ranks.
His support is also reminiscent of the ongoing backing Waugh offered Langer through his career, when he often found himself under pressure despite finishing with a Test average of 45.27
Waugh once famously responded to a question from a reporter of what advice he would offer Langer in 1999 with: "To stop reading your shit".
Langer averaged just 32.65 at the time, and responded with a match-winning innings in Hobart.
While Langer wasn't quite as blunt as Waugh on Thursday, he has made no secret that he believes Harris can succeed at the top.
"That's absolutely one of the most important things in life, knowing people have got your back," Langer said.
"My experience, when Steve Waugh, Ricky Ponting, Mark Taylor or Allan Border said 'you're in the team', you feel like you feel like Superman.
"You feel like you're important to the team and Marcus Harris is important to the team."
Australia's faith in Harris is made even more important by the fact Warner has had 13 opening partners in his 10-year Test career.
"I think an opening partnership is very, very important," Langer said.
"We're really confident Marcus has got what it takes to be a successful Australian opening batsman.
"And what we see in the nets, what we see in domestic cricket all adds up to what is potentially a very good Test career."
"One of the building blocks of a great team is the opening partnership and top three.
"We're determined to get that right."
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