William John Brewer (c. 1843 - June 19, 1878) was an American soldier who received the Medal of Honor for valor during the American Civil War.

Brewer served in the Union Army in the 2nd New York Cavalry.

He received the Medal of Honor on May 3, 1865, for his actions during the Appomattox campaign.

Citation: Capture of engineer flag, Army of Northern Virginia.

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