It was not immediately clear what caused the fire at the facility, which was decimated by the explosion on August 4 when nearly 3,000 tons of ammonium nitrates detonated.
A column of black smoke billowed from the port with orange flames leaping from the ground.
The explosion in August killed more than 190 people, injured around 6,500 and damaged thousands of buildings in the Lebanese capital.
The sight of another huge fire a month later created panic among residents traumatised by last month’s explosion.
The state-run National News Agency said the fire was at a warehouse where tyres are placed. It added that firefighters are dealing with the blaze.
Army helicopters were also taking part in efforts to extinguish the fire.
Residents cracked open windows and called each other to warn of the new fire.
Local TV stations said companies that have offices near the port have asked their employees to leave the area.
Lebanese troops closed the major road that passes near the port, referring traffic to other areas.
