John Badger Bachelder (September 29, 1825 – December 22, 1894) was a portrait and landscape painter, lithographer, and photographer, but best known as the preeminent 19th-century historian of the Battle of Gettysburg in the American Civil War.

From his brief association with military topics, he retained a lifelong interest in them, and when the Civil War began in 1861, he was already collecting notes on Bunker Hill, planning to paint an accurate rendition of the battle.

In 1870, noted artist James Walker was commissioned by Bachelder to paint an account of Pickett's Charge, entitled "The Repulse of Longstreet's Assault at the Battle of Gettysburg," a massive work that measured 7.5 by 20 feet.

He coined the phrase "Copse of Trees" and invented the concept of a "High Water Mark of the Confederacy" at which the famous Pickett's Charge peaked.

Bachelder's activities at Gettysburg were described in detail by Senator Wade Hampton of South Carolina on March 17, 1880, in a report to the Senate from its Military Affairs Committee: It appears that Mr. Bachelder, having the advantage of a military education, and love of history, went to the front early in 1862, more than a year before the battle of Gettysburg, to be in a position to collect data when the most important battle of the War was fought.

With this information and sketches thus secured, he visited the Army of the Potomac, spending the winter of 1863–64 in consultation with the officers of every regiment and battery, whose conversations and explanations were carefully noted and preserved.

At the close of the war, Mr. Bachelder issued an invitation to the many officers whose acquaintance he had made to visit Gettysburg with him for historical purposes, which was accepted by over one thousand; forty-nine of them generals commanding.

From the acquaintance thus secured has resulted, during the past sixteen years, a most valuable correspondence regarding the battle of Gettysburg.Bachelder died of pneumonia in Hyde Park, Massachusetts, in 1894.