"There is a resolution from March 25 ... a favourable ruling," Judge Nery Kunzle told reporters in Asuncion referring to the 25-year-old's request for Paraguayan nationality through his mother.
Barrios, on course to be crowned the Bundesliga's top scorer, said the news was a happy surprise although he played down the chances of a call from Paraguay's Argentine coach Gerardo Martino.
"I had no idea ... I'm a bit surprised and at the same time feel proud if what's happening comes true," Barrios, who made his reputation with Colo Colo in Chile and Mexico's Atlas, told Primero de Marzo radio from Germany.
Kunzle said the request was put through by Sergio Godoy, a lawyer with close links to the Paraguayan Football Association national teams director Horacio Cartes.
At his club, Barrios, who joined Dortmund last year, plays alongside Nelson Haedo, a striker who carries a large part of Paraguay's World Cup hopes after Salvador Cabanas was shot in the head in Mexico City in January.
Martino already has two Argentine-born players in his squad, midfielders Jonathan Santana, a regular in the qualifiers, and Nestor Ortigoza, naturalised late last year.
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