According to Hegseth, three drug-smuggling "terrorists" from a Colombian left-wing rebel group were killed in the attack, which is said to have taken place on October 17 in international waters.
Exactly where it took place is not clear.
This is the latest in a series of attacks on boats that, according to the American administration, smuggle drugs to the USA.
In total, at least 30 people have been killed in the attacks, which have mostly taken place off the coast of Venezuela. On Wednesday, US President Donald Trump gave the intelligence agency CIA the green light for secret operations inside Venezuela.
Trump has also made an enemy of Colombia's President Gustavo Petro, who accuses the USA of having violated the country's sovereignty in an attack on a boat in September in which an innocent fisherman is said to have been killed.
Trump has responded by calling Petro a "drug leader".
Corrected version: In an earlier version of the text, an incorrect quote appeared.