Mike Hancock told the BBC he was supporting Katia Zatuliveter despite reports that MI5 had ordered her arrest on espionage charges.
"Nobody has shown me any evidence to support the view that she is any way a threat to the U.K.," said Hancock, a member of the Liberal Democrats.
The Sunday Times cited security sources saying the 24-year-old Zatuliveter would be expelled from Britain.
Zatuliveter earned a master's degree at a British university and began working in Hancock's office in 2008. Hancock's district includes the naval base at South Portsmouth, but Hancock says his dealings with the Royal Navy did not involved particularly sensitive information.
The BBC said the young woman had been on investigators' radar since last summer.
"If she was a threat, when they stopped her in August, they could have removed her then," Hancock said.