Louis J. Weichmann (September 29, 1842 – June 5, 1902) was an American clerk who was one of the chief witnesses for the prosecution in the trial following the assassination of Abraham Lincoln.

He wrote in his autobiographical work, A True History of the Assassination of Abraham Lincoln and of the Conspiracy of 1865, that he desired to pursue a career as a pharmacist, but, at the behest of his mother, he reluctantly agreed to study for the Roman Catholic priesthood.

Weichmann testified that on the day President Lincoln was shot, April 14, 1865, he accompanied Mary Surratt to her other property in Surrattsville, (now Clinton, Maryland), where she delivered items that Booth later retrieved hours after the assassination.

Samuel Mudd, the doctor who treated Booth's broken leg on the night Lincoln was killed, and claimed to have no knowledge of the conspiracy, was linked by Weichmann's testimony to the events for which he was tried and found guilty as well.

Augustus Howell, a blockade runner who worked with John Surratt, claimed during the trial that Weichmann provided the Confederates with classified information obtained by his position at the War Department.

[2] Partially because of this, he swore out an affidavit, shortly before his death, reaffirming that all his testimony concerning Abraham Lincoln's assassination was completely true.