The suspect was detained on Monday morning as he was making his way to his job at Tromso university, in northern Norway, public broadcaster NRK said.
“We think he’s working undercover for the Russian authorities,” PST deputy director Hedvig Moe told broadcaster TV2.
“An undercover agent is someone who gathers information, most often for his or her country of origin — in this case Russia and the Russian intelligence services,” she said.
The PST, which said it had been coordinating with intelligence services from allied nations, wants the suspect expelled from Norway. In the meantime, he has been slapped with a four-week detention order.
The alleged spy worked at Tromso university on Norway’s Arctic policy and “hybrid” threats, the PST said.
The latest affair follows a rash of arrests of Russian citizens accused of flying drones over Norwegian territory in violation of a ban imposed in the wake of Moscow’s invasion of Ukraine.
Other Russians have been detained in Norway on charges of illegally photographing sensitive sites. Norway is a member of the NATO military alliance.
In the Arctic Circle, it shares a 198-kilometre (123-mile) border with Russia, which it has displaced as Europe’s top supplier of natural gas.