Yes, all the films, including The Hateful Eight , are a part of the same cinematic universe.
In an interview released just seventeen hours ago , Tarantino clarified that the films Kill Bill and From Dusk Til Dawn are the film within the films from the "Real Universe" that the characters watch.
The "Real Universe" is the cinematic world where everything takes place in the same setting.
According to Tarantino, in the article published by Dazed Digital : There is actually two separate universes.
There is the realer than real universe, alright, and all the characters inhabit that one.
But then there's this movie universe.
So From Dusk Till Dawn , Kill Bill , they all take place in this special movie universe.
So when all the characters of Reservoir Dogs or Pulp Fiction , when they go to the movies, Kill Bill is what they go to see.
From Dusk
Till Dawn is what they see.
In an updated version of that same article, Tarantino confirmed that at least one of the characters in
The Hateful Eight is in fact related to a character from a previous film, which takes place further along the timeline.
I don’t want to say it point blank ‘boom’,
but if you’re a fan of my work and are familiar with it you could be a bit of a detective.
I will say that there is one character amongst the eight that is related to one of my other characters in one of my other films.
As for the identity, Tarantino has not affirmatively confirmed, but the strongest clues hint at Tim Roth's Oswaldo Mobray, aka Pete Hicox , who shares the same legal name with Michael Fassbender's Archie Hicox in Inglourious Basterds .
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Tim Roth's character in The Hateful Eight is apparently an ancestor of one of the Inglourious Basterds.
From the ew.com article, How Quentin Tarantino's Hateful Eight links to Inglourious Basterds : Tim Roth ( Reservoir Dogs , Pulp Fiction ) plays Oswaldo
“The Little Man” Mobray in the post-Civil War western, and the actor recently told the Huffington Post that the character is linked to someone from another Tarantino film.
“I won’t say which one,” Roth said.
“He’s the great-great-grandfather of one of the characters in Inglourious Basterds .”
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At some point, Bob smokes a Manzana Roja ( Red Apple in Spanish) cigarette, and Minnie has a cigarette with Red Apple tobacco.
This Fictional brand has appeared before in other Tarantino films, although I think this is the first time it's suggested to have a foothold in the Spanish-speaking market.
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To add to Carl Fink's answer (don't have enough reputation to comment), Tim Roth's character's real name is "English" Pete Hicox, while Michael Fassbender's very British character in Inglourious Basterds is Archie Hicox.
That's almost certainly the connection, or at least one of them.
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Yes,it is indeed a part of the "Tarantino Cinematic Universe", as is evident from the following points:
Mexican Bob smokes a Manzana Roja cigarette which translates to Red Apple cigarette, the infamous Tarantino cigarettes .
Miny smokes Red Apple tobacco, another Red Apple reference.
Oswaldo Mobray whose real name is English Pete Hicox, is the grandfather of Lt. Archie Hicox from Inglourious Basterds .
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It is well known that the majority of Tarantino movies are set in the same fictional Universe. I just wonder what are, if any, the cross-references that make "The Hateful Eight" part of the same puzzle.  In one of the answers to this question -  "How many Tarantino films are set in the same universe? " - it is mentioned that "The Hateful Eight" is indeed the part of that universe. But I guess it would be nice to have some clarification on this issue.
Does "The Hateful Eight" belong to the Tarantino Universe?
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<quentin-tarantino><the-hateful-eight>
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The Hateful Eight is part of the Tarantino universe as confirmed by Tarantino himself. All his films are set in the same universe. he Hateful Eight is in fact related to a character from a previous film, which takes place further along the timeline. There are two different universes. 
Tarantino explained the true names of the movies his characters watch. There are two different universes. The "Real World" is a cinematic setting in which everything occurs. There is a "realer" universe and all the characters reside in.