Sylvester Bonnaffon Jr. (September 14, 1844 – May 12, 1922) was a United States soldier who fought with the Union Army during the American Civil War as a first lieutenant with Company G of the 99th Pennsylvania Infantry.

[1] He received his nation's highest award for valor, the U.S. Medal of Honor, for his gallantry and leadership against Confederate troops during the Battle of Boydton Plank Road, Virginia on October 27, 1864.

[2] Also brevetted twice for "gallant and meritorious services during the War", he enrolled with the Pennsylvania National Guard post-war, rising through the ranks to command the 20th Regiment Infantry (emergency) during the Pittsburgh railroad strike of 1877.

Engaged with his regiment in the Siege of Petersburg, including the First and Second Battles of Deep Bottom (July 27–29, 1864 and August 14–20), he was then advanced in rank again with a commission to first lieutenant on September 27.

[9] While leading his men in the Battle of Boydton Plank Road, Virginia on October 27, 1864, as a member of Brigadier-General Regis De Trobriand's staff (2nd Corps, 1st Brigade, 3rd Division), he was severely wounded in action as he rallied his troops "in the face of a terrible fire of musketry", according to his Medal of Honor citation.

[20][21][22] Earlier that same year, he was preceded in death by his brother, Charles, a long-time jeweler in Philadelphia who was fatally burned in a fire on January 29, 1920, caused by smoking in bed.