The breach affected a DNC data analytics programme used by the campaign and a number of other organisations, according to the campaign.
It said outside security experts reviewing the campaign's computer system found "no evidence that our internal systems have been compromised".
The brief statement did not specify what types of data the service was analysing, but partnerships with modern e-commerce companies can allow sophisticated tracking, categorisation and identification of website visitors. This can help organisations tailor their online content, advertising and solicitations to be more effective.
The announcement comes as the FBI investigates a hack at the DNC that resulted in the posting last week of embarrassing internal communications on WikiLeaks, and a similar intrusion of the House of Representatives Democratic Campaign Committee.
President Barack Obama has said Russia was almost certainly responsible for the DNC hack, an assertion with which cybersecurity experts have agreed.
The FBI said on Friday it was aware of "media reporting on cyber intrusions involving multiple political entities, and is working to determine the accuracy, nature and scope of these matters".
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The intrusions have added another layer of mystery to the hacking of Democratic Party information that has been revealed in the heat of this year's presidential and congressional elections.
