Specialist Sean Walsh from San Jose was killed by "indirect fire" in the Khowst Province, the Department of Defense announced on Friday.
The 21-year-old was deployed to Afghanistan last December. Officials say he is the second member of the California National Guard to be killed in action in Afghanistan.
The Guard's Adjutant General, Major General David Baldwin, called Walsh a warrior whose service exemplified the bravery and sacrifice of the state's "citizen-soldiers."
The Associated Press contributed to this story.