He was arrested by Soviet forces in Hungary in 1945 and disappeared. He is thought to have died in a Soviet prison, but his fate remains a mystery.
Thousands of Jews escaped deportation to Auschwitz because Wallenberg gave them Swedish protective passes.
His story became legendary after 1945.
The former Soviet authorities said he must have died in a Moscow prison in 1947.
But his family were sceptical about that version of events, and spent decades trying to establish what actually happened to him.
Last November they called on the Swedish Tax Agency to officially declare him dead. The request was made via Sweden's SEB Bank, acting as a trustee.
"He shall be deemed to have died 31 July, 1952," the tax agency said.
Pia Gustafsson of the tax agency said that date was chosen because it fell "five years after he went missing, which was believed to be the end of July, 1947".
That procedure complied with a Swedish law used in cases where the circumstances of death were not clear, she told the BBC.
His family requested that he be declared dead officially, in order to "let Raoul rest in peace", Sweden's Aftonbladet daily reported.
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In August the World Jewish Congress quoted a report alleging that Wallenberg was executed in a Soviet prison in 1947. The claim was made in a diary by Ivan Serov, a former director of the Soviet KGB secret police.
The Israeli Holocaust memorial at Yad Vashem honours Wallenberg as one of the "righteous among the nations" for having saved so many Jewish lives in the war.
Wallenberg's rescue efforts began in July 1944, after his arrival as a diplomat in Budapest. The Nazis had already deported nearly 440,000 Jews from Hungary, most of them to the Auschwitz death camp, where many were murdered on arrival.
Ms Gustafsson said two contradictory Russian statements had contributed to Sweden's decision to declare Wallenberg dead:
Wallenberg's mother Maj von Dardel and stepfather Fredrik von Dardel committed suicide in 1979, heartbroken over the Soviet authorities' failure to disclose details of Wallenberg's fate, the Wall Street Journal reported.
Swedish diplomat Raoul Wallenberg, who helped thousands of Hungarian Jews to escape Nazi mass murder in World War Two, has been formally pronounced dead by Sweden's Tax Agency.