About 67 containers were in the water, the U.S. Coast Guard said on the social platform X.
Long Beach, about 20 miles (32 kilometers) south of Los Angeles, is one of the busiest seaports in the country, with 40% of all shipping containers in the United States coming through it or the Los Angeles port.
Some of the containers appeared to have fallen on the STAX 2, an anti-pollution vessel attached to the side of the Mississippi that captures emissions. When empty, a container can weigh between two to four metric tons (2.2 to 4.4 tons) depending on the size.
The Pier G container terminal, one of six at the port, temporarily stopped unloading and loading ships as authorities worked to secure the containers.
The Mississippi sails under the flag of Portugal and arrived in Long Beach after departing Aug. 26 from the Yantian port in Shenzhen, China, according to vessel tracking websites.