Dennis Gahagan (born c.1817) was an immigrant in the Mexican Pueblo de San Diego in territorial California, and early settler in the City of San Diego in the new U.S. state of California.
He arrived in San Diego by 1849 as a soldier with the First U.S.
He and his fellow troops fought and patrolled in the Southern California countryside during the war.
In 1849 Gahagan was elected U.S. Judge of the 1st Instance of the District of San Diego.
From 1849 to 1850 Gahagan was an alcalde (mayor) of San Diego.