The physicist Dmitri Kolker, accused of high treason, died in Moscow a few days after his imprisonment.
He was transferred from the detention center to the intensive care unit of a Moscow hospital and died there, the Interfax news agency said on Sunday.
Kolker had headed the Laboratory for Quantum Optics Technologies at the Institute for Laser Physics at Novosibirsk State University.
Quantum optics deals with the interaction between light and matter.
Kolker was involved in the development of lasers for medical purposes, among other things.
The 54-year-old physicist was suffering from pancreatic cancer in the final stages and was arrested a few days ago by the FSB secret service in a Novosibirsk hospital and taken to Moscow.
Investigators accused him of betraying secrets to China.
According to his son, the researcher gave lectures to students as part of a science exchange in China, but was always accompanied by FSB agents.
The lectures were also coordinated down to the last detail with the secret service, so that no information that was considered a state secret appeared there, he said.
In addition to Kolker, 75-year-old Anatoly Maslow, who was researching hypersonics, was also arrested in this connection.
The prosecution of scientists for alleged high treason has recently increased significantly in Russia;
since the beginning of the war of aggression against Ukraine there have been several cases.