The Lebanese army found 4.35 tonnes of ammonium nitrate near the entrance to Beirut's port weeks after a huge explosion of the same chemical killed dozens of people and injured a further 5,000 on 4 August, the NNA state news agency reported.
According to the army's statement, quoted by the agency, army engineers were dealing with the chemical.
Meanwhile, a rescue worker told reporters on Thursday that his colleagues have detected signs of life under the rubble, almost a month after the explosion hit the Lebanese capital.
"These (signs of breathing and pulse) along with the temperature sensor means there is a possibility of life," rescue worker Eddy Bitar explained.
At least 190 people were killed and 5,000 more injured when two explosions rocked the port of Beirut on 4 August, also causing huge damage to several parts of the city. 300,000 people were left homeless.
The authorities blamed the explosion on the unsafe storage of more than 2,500 tonnes of highly explosive ammonium nitrate in a port warehouse.
The deadly blasts triggered mass protests in Lebanon, with thousands of people taking to the streets to demand the government's resignation. The tragic incident has aggravated the country's economic crisis and added fuel to anti-government sentiment, forcing the prime minister and his cabinet to step down.
Last week, Lebanon's former ambassador to Germany, Mustapha Adib, was named the new prime minister-designate with the backing of most MPs after being nominated by the Shi'ite group Hezbollah.
On 9 August, world leaders pledged nearly $300 million to Lebanon at the UN-backed donor conference.