Huber Gomez Patino was arrested in the town of Arteaga, in the western state of Michoacan, a stronghold of the gang, the office of Mexico's federal police said in a statement.
The man "said that he was the son of Servando Gomez Martinez, and threatened federal agents with assassination if they did not release him," the statement said.
Gomez Patino had a weapon and half a kilo of a synthetic drug, but "was captured before he could put up any resistance," the statement added.
Another honcho, Enrique "Kike" Plancarte, was killed by troops in late March. His uncle, Dionisio Plancarte, who was also a key leader of the gang, was detained in January.
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The Knights Templar gang is a spin-off of the once dominant cartel in the region known as La Familia Michoacana.
Moreno founded La Familia, but the gang broke up after the government claimed to have killed him in 2010.
Civilians formed vigilante forces in February 2013 to chase the cartel out of their towns, accusing local police of either turning a blind eye or colluding with the gang.