Dotcom had asked the High Court to review the legality of an official's August 2024 decision that he should be surrendered to the US to face trial on charges of copyright infringement, money laundering and racketeering.
It was the latest chapter in a protracted 13-year battle by the US government to extradite the Finnish-German millionaire from New Zealand.
The Megaupload founder had applied for what in New Zealand is called a judicial review, in which a judge is asked to evaluate whether an official's decision was lawful.
A judge on Wednesday dismissed Dotcom's arguments that the decision to deport him was politically motivated and that he would face grossly disproportionate treatment in the US.
In a written ruling, Justice Christine Grice also rejected Dotcom's claim that New Zealand police were wrong to charge his business partners, but not him, under domestic laws - which likely yielded laxer sentences than if the men had been tried in the US.
The latest decision could be challenged in the Court of Appeal, where a deadline for filing is October 8.
One of his lawyers, Ron Mansfield, told Radio New Zealand that Dotcom's team had "much fight left in us as we seek to secure a fair outcome", but did not elaborate further.
Neither Dotcom nor Mansfield responded to a request for comment from The Associated Press on Thursday.
The saga stretches to the 2012 arrest by New Zealand authorities of Dotcom in a dramatic raid on his Auckland mansion along with other company officers at the request of the FBI.
US prosecutors said Megaupload raked in at least $US175 million, mainly from people who used the site to illegally download songs, television shows and movies, before the FBI shut it down earlier that year.
Lawyers for Dotcom and the others arrested argued it was the users of the site, founded in 2005, who chose to pirate material, not its founders.
But prosecutors said the men were the architects of a vast criminal enterprise, with the Department of Justice describing it as the largest criminal copyright case in US history.
He has been free on bail in New Zealand since February 2012.
