"We have confirmed by phone that he is released," Mattias Robertson of the Swedish Armed Forces told Swedish Radio News.
Reuters reports that Johansson was freed for health reasons, as he suffers from a mild form of diabetes.
Johansson and his seven European colleagues were on a mission in eastern Ukraine when they were detained Friday by pro-Russian separatists in the town of Slaviansk. As of Monday morning, Johansson's fellow observers - from Germany, Poland, the Czech Republic and Denmark - were still detained.
Foreign Minister Carl Bildt welcomed the news, writing on Twitter that learning about Johansson release was "one of the best phone calls for a long time."