The 29-year-old research student at the Swedish University of Agricultural Sciences in Uppsala, was arrested on February 15 in Uppsala.
He was initially ordered to remain in custody until March 3, but that deadline was extended by two weeks, before being extended once more on Thursday.
A decision whether to indict him will now be taken by March 30, Stockholm court secretary Karin Hoffström told AFP.
Swedish prosecutor Tomas Lindstrand said last month that the Russian, who studied plants and plant viruses, had been spying on Sweden since January 2005 and that the espionage had harmed Sweden’s “overall defence”.
He did not say which country had received information from the 29 year old.
On Thursday, Lindstrand added that the suspect was also suspected of “illicit intelligence gathering”, a lesser charge than espionage.
Russia on Wednesday summoned the Swedish ambassador to demand the release from custody of the suspect, and warned that the case threatened relations between the two countries.
“The Russian side insists on his immediate release,” the Russian foreign ministry said in a statement.
“The conduct of the Swedish authorities in this affair will not fail to have an impact on overall Russian-Swedish relations,” it added.
The suspect risks up to six years in prison if convicted of spying.