"The Eiffel Tower has been closed and is in the process of being evacuated because a person is scaling it," a spokeswoman for the landmark said.
Firefighters are at the scene and have established contact with the man climbing the tower, a spokesman for the fire brigade said.
Irish tourist Claire Murphy, who is on vacation in Paris, told CNN that the lockdown began at 2:15 p.m. local time and that she was still stuck on the tower's top floor.
Almost seven million people visit the Eiffel Tower every year. Completed in 1889, it was designed and built by Gustave Eiffel as part of a "Universal Exposition" marking the centenary of the French Revolution. It celebrated its 130th anniversary last week.
This isn't the first time someone has tried to scale the tower -- three people were spotted climbing the landmark in 2015. It was also evacuated twice in 2010 due to bomb threats, once in 2013 over security concerns and was closed for two days following theterror attacks in 2015.
In 2017, a protective glass wall, 65 millimeters thick and resistant to bullets, was erected around the base of the tower to provide increased security.