US District Judge Dana Christensen found that Governor Steve Bullock, a Democrat, was within his authority to allow the state's counties to use a mail ballot system to avoid the risk of transmitting COVID-19 at polling places.
The Trump campaign was among a number of plaintiffs that challenged the governor's move.
The White House did not immediately respond to a request for comment.
Voting by mail has become a flashpoint in the Nov. 3 election, with the Republican president making unfounded complaints that the process leads to widespread election fraud, a point he reasserted in Tuesday's debate with Democratic challenger Joe Biden.
"The evidence suggests, however, that this allegation, specifically in Montana, is a fiction," wrote Christensen, who was nominated by former Democratic President Barack Obama.
The judge also noted that Montana used mail voting in its June primary and had a record voter turnout without evidence of fraud.
Trump's lawsuits described the governor's directive that allows a ballot to be sent to every voter as a "brazen power grab."
Polls show Montana to be leaning for Trump in the presidential election, but Bullock is running a tight race for the USSenate against Republican incumbent Steve Daines.
Voters will also be selecting a new governor in November.
Republicans have challenged various aspects of mail voting in courts across the country. Judges have ruled for Democrats in battleground states, including Wisconsin, Michigan and Pennsylvania, where courts this month held that mailed ballots arriving within certain time periods after Election Day must still be counted.