Trump's re-election campaign filed the lawsuit in August.
U.S. District Judge Michael Shipp wrote that the plaintiffs "fail to establish they are likely to succeed on the merits of their claims."
"It is difficult - and ultimately speculative - to predict future injury from evidence of past injury," Shipp wrote on Thursday.
"We are disappointed with the ruling and are assessing our options," Republican National Committee National Press Secretary Mandi Merritt said in a statement cited by Bloomberg News.
New Jersey Governor Phil Murphy's decision in August came as Trump, a Republican, stepped up his attacks on voting by mail, which is expected to increase dramatically because of the novel coronavirus.
The Trump campaign's lawsuit made two charges - first, that Murphy exercised power that belonged to the state legislature in changing the state's election law, and second, that the changes "will violate eligible citizens' right to vote".
Trump has railed against mail-in voting, making accusations that it leads to fraud. Experts have said such fraud is rare.