The death has sparked public health fears on social media but the authorities said on Monday the situation was under control.
Plague is endemic to Siberia, location of the first known prehistoric human outbreak, as the bacterium that causes it - Yersinia pestis - is present among the small wild mammals in its vast wilderness and their fleas.
It is rare for Russians to fall ill with plague, however, although cases in other parts of the world - notably Africa - occur every year.
Local media named the woman as Darya Shipilova, an employee at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East.
There were conflicting reports about how and where the laboratory worker, whose age was given as 28, had fallen ill but the authorities said she had died of pneumonia of as-yet unestablished origin.
Russia's health watchdog Rospotrebnadzor told state media the institute had not found evidence of any accidents involving the handling of pathogens after rumours spread on social media that the woman had dropped a test tube containing live bacteria.
Nevertheless, Anna Popova, the head of Rospotrebnadzor, flew to Siberia for an emergency meeting over the weekend.
An unspecified number of people who had been in contact with the dead woman were quarantined, parts of the hospital in the town of Shelekhov where she died were cordoned off and an aluminium plant told its workers to wear masks.
The hospital has been placed under quarantine, Mayor Maxim Modin said on Friday, no longer admitting or discharging patients, and its laboratory halted carrying out routine tests.
The hospital's outpatient clinic kept operating as usual, the mayor said.
The Irkutsk Aluminum Plant in Shelekhov required staff to wear masks as a precaution, its director Artyom Fominykh said on Telegram, calling for calm and adding: "Better be safe than sorry."
Authorities urged people to remain calm and said there was no evidence that the woman's death was linked to her work.
Igor Kobzev, the governor of the Irkutsk region, said in a statement that she had died of "pneumonia of unknown aetiology," adding: "Importantly, over the past few days, no new cases of staff from the Anti-Plague Institute seeking medical attention have been identified."
He said the situation was under control and that all known contacts of the dead woman had been identified, examined and placed under medical supervision.
Two people who were in contact with the woman tested positive for a virus that causes a common cold and two others for COVID-19, Russia's health agency said, adding that others who were tested were all negative.
Independent local news outlet Lyudi Baikala quoted sources it did not identify as saying the woman died from pneumonic plague and that several Irkutsk hospitals were placed under quarantine.
Asked on Monday to comment on the case, Kremlin spokesman Dmitry Peskov said the public and media should follow official statements from Rospotrebnadzor rather than "rumours or speculation".
US Secretary of State Marco Rubio said on Monday the United States was closely monitoring the case involving the laboratory worker.
"We're watching and monitoring it closely," Rubio told reporters before leaving for a trip to Europe.
"I don't think it's cause for alarm but it is cause for focus and a cause just to keep an eye on it."
Plague comes in three forms affecting different parts of the body.
Pneumonic plague, an infection of the lungs, prompts the patient to cough out infectious airborne droplets.
If identified quickly, it can be treated with antibiotics.
But if left untreated, it can be fatal in one to six days.
with AP
