The breach, which was first reported by the "Washington Post" on Tuesday, was confirmed by the chair of the Democratic National Committee (DNC), Debbie Wasserman Schultz.
Wasserman Schultz said the hackers had been ejected from the database as soon as the intrusion became apparent.
"When we discovered the intrusion, we treated this like the serious incident it is ...," she said in a statement. "Our team moved as quickly as possible to kick out the intruders and secure our network."
Some of the hackers are thought to have had access to the Democratic National Committee for about a year, with all believed to have now been expelled.
The Washington Post had reported that the Russian spies had gained access to the entire database of opposition research on Trump.
"The intruders so thoroughly compromised the DNC's system they also were able to read all email and chat traffic," the paper said, citing committee officials and security experts.
Other networks hacked
The DNC stressed that no financial, donor or personal information had been accessed, which would suggest traditional espionage rather than the actions of criminal hackers.
Robert Deitz, a former general counsel at the National Security Agency, said the purpose was to understand the "proclivities" of intelligence targets.
"Trump's foreign investments, for example, would be relevant to understanding how he would deal with countries where he has those investments," should he be elected, Deitz said. "In short, this sort of intelligence could be used by Russia, for example, to indicate where it can get away with foreign adventurism."
The Director of US National Intelligence, James Clapper, said last month that US presidential campaigns were threatened by hacks from those bent on espionage as well as other activities such as political mischief.
The latest report comes only a day after Trump said he was revoking press credentials from the Washington Post over a headline he disagreed with.
rc/msh (AP, Reuters)