More than 4,000 Roma are included in the registry, the Dagens Nyheter (DN) newspaper reports.
Police confirmed that the registry exists on Monday morning, after initially denying it, but added that the registry was not sanctioned by the police. Investigations have been launched to determine whether the list was compiled by a single employee.
According to DN, the database consists of a family tree that includes personal identity numbers (personnummer) and addresses combined with arrows that indicate how people included in the registry are related.
The registry is reportedly kept in a computer file folder labelled "itinerants" (kringresande) within the computer system of police in Skåne in southern Sweden.
According to lawyers interviewed by the newspaper, the database breaks a number of laws, including the European Convention on Human Rights, police data laws, and the law against general police surveillance registries.
"Is 'itinerants' a euphemism for being Roma and if so a registry based on ethnic background? It's prohibited to create a registry solely based on those grounds," constitutional law expert Johan Hirschfeldt told DN.
The computer file seen by the newspaper was created on May 29th, 2012, but the information in it is believed to be older. According to DN, police actively use the database in their current operations.
"Why are the police keeping tabs on Roma? It's racism. It's creepy that my children are in there," said Sandra Häkansson, whose entire family appears in the database.
According to DN, the registry was created by police in Lund in southern Sweden, but Skåne County police, the National Bureau of Investigation in Stockholm, and law enforcement intelligence agencies in several cities have access to it.
Lars Förstell, a spokesman with the Skåne police, initially questioned whether the database exists.
"There is no basis for those claims. It sounds totally inappropriate. I was baffled when I heard about it. Now I've spoken with others who are also as baffled as me and asked the same question I did: what the heck would we use it for?" he told Expressen.
Later on Monday, Förstell confirmed that the registry did indeed exist and that an investigation was under way.
"A record for the sole purpose of describing the Roma and their relations with each other is not in line with our conditions and is not something that has not been commissioned by the authorities," he told the TT news agency.