The ruling was delivered amid a case involving the right-wing channel PragerU, which claimed that the video-sharing platform was guilty of infringing the US constitution's First Amendment by "censoring" its opinions.
However, the ruling - which could set a new precedent in the US - concluded that the First Amendment does not apply to the Google-owned company.
In response to the verdict, PragerU insisted it was still "not done fighting for free speech".
The First Amendment, which dates back to 1791, was intended to apply between the US government and its citizens.
However, there have been some rare instances when it has been applied to companies, too.
But, in this particular case, the court said: "Despite YouTube's ubiquity and its role as a public-facing platform, it remains a private forum, not a public forum."
PragerU's Craig Strazzeri has refused to accept the verdict and has promised to continue to fight the ruling.
He said: "Of course this ruling is disappointing, but we won't stop fighting and spreading public awareness of Big Tech's censorship of conservative ideas."