Count Jacob Johan Anckarström (May 11, 1762 - April 27, 1792) was a Swedish soldier, and regicide. He served as a Captain in the King Gustav III's Regiment between 1778 and 1783. During travels on Gotland, he was accused of slandering the King and fled to Stockholm, where he spent the winter, but was subsequently arrested, brought back and tried in Gotland. Although he was acquitted due to lack of evidence, he later maintained in his confession that this incident sparked his fire of hatred towards the King, fuelled by the contemporary revolutionary movement in Europe.

On March 16, 1792, King Gustav III had returned to Stockholm, after spending the day at Haga Palace outside the city, to dine and visit a masquerade ball at the Royal Opera. During dinner, he received an anonymous letter that contained threat to his life, but as the king had received numerous threatening letters in the past, he chose to ignore the warning. After dining, he left his rooms to take part in the masquerade. Soon after entering, he was surrounded by Anckarström and his co-conspirators Claes Horn and Adolf Ribbing , wearing black masks, greeting him in French with the words "Bonjour, beau masque" ("Good-day beautiful mask"). Anckarström moved in behind the King and fired a pistol-shot into the left side of his back. The King jumped aside, crying in French "Ah! Je suis blessé, tirez-moi d'ici et arrêtez-le" ("Ah! I am wounded, take me away from here and stop him!") The King was immediately carried back to his quarters, and the exits of the Opera were sealed. Anckarström was arrested the following morning, and immediately confessed to the murder, although denying a conspiracy until informed that Horn and Ribbing also had been arrested and confessed in full.

Gustav III died of his wounds on March 29 and on April 16 Anckarström was sentenced. He was stripped of his estates and nobility privileges. He was sentenced to be cast in irons for three days and flogged, and his right hand was to be cut off before he was decapitated. The execution took place on April 27, 1792.

In the same year the Anckarström family changed its surname to Löwenström and donated funds for a hospital as a gift of appeasement. This resulted in the Löwenström Hospital , or Löwenströmska sjukhuset. Living descendants of Anckarström include Ulf Adelsohn.

See also: Un Ballo in Maschera