The DNC's computer network was infiltrated in a "serious" incident, said chairwoman Debbie Wasserman Schultz.
A cyber security firm has been hired to help, she said, to "kick out the intruders and secure our network".
Russia has denied any involvement in the hacking.
"I completely rule out a possibility that the [Russian] government or the government bodies have been involved in this," Dmitry Peskov, a Krelim spokesman, told Reuters in Moscow.
