The figure, which is more than any Democratic candidate raised in the last three months of 2019, came as Mr Sanders’ rivals in the race showed signs of financial strain in recent days.
Former vice president Joe Biden and Pete Buttigieg, former mayor of South Bend, Indiana, have both taken time out from the campaign trail to raise money just days before the New Hampshire primary, according to The New York Times.
Mr Sanders came out of the Iowa caucus, the first contest of the Democratic primary, on Monday just behind Mr Buttigieg in terms of state delegate equivalents (SDEs) – the traditional measure for success in the state – with most of the ballots counted.
With 97 per cent of precincts in Iowa reporting, Mr Buttigieg narrowly led Mr Sanders by 550 SDEs to 547, although Mr Sanders led in terms of the public vote (by 26.5 per cent to 25 per cent).
Delays to the release of Iowa’s results has meant a clear winner has not yet emerged from the first contest, which has often been influential in establishing the frontrunner for the Democratic nomination.
The Vermont senator’s $25m haul was his best in a single month of this campaign and follows a year in which he raised $96m, more than any other Democratic candidate.
Mr Sanders’ campaign has announced it has sent more staff to the 14 states which are due to vote on 3 March, commonly known as Super Tuesday.
“Working-class Americans giving $18 at a time are putting our campaign in a strong position to compete in states all over the map,” Faiz Shakir, Mr Sanders' campaign manager, said in a statement on the fundraising haul.
The statement did not say how many staff members would be deployed to these states.
A strong fundraising operation will be important for whichever Democratic candidate takes on Donald Trump, who raised a total of $143m in 2019, in the November election.
The other 10 Democrats running for president have not yet announced their January fundraising figures.
However, billionaire businessman Michael Bloomberg, who is funding his campaign from his personal fortune, said on Tuesday he would increase his staff to 2,100 people and double TV ad spending after the Iowa caucus.
Mr Bloomberg, who only declared his candidacy in November and has opted to skip early voting states, has already spent more than a quarter of a billion dollars on advertising which has mainly targeting Super Tuesday states.
Additional reporting by Reuters