Antonie von Birkenstock Brentano, (Vienna, April 28, 1780 – April 12, 1869, Frankfurt am Main) is notable as being one of the likelier of the many possible candidates put forward by scholars as composer Ludwig van Beethoven's Unsterbliche Geliebte, or Letter To The Immortal Beloved. Beethoven dedicated the Diabelli Variations Op. 120 to her, and his piano sonata Op. 90 to Maximiliane, her daughter. Her handwriting has been identified on a manuscript copy of the song cycle, An die ferne Geliebte (To the Distant Beloved), in which the following is written: "Requested by me from the author on March 2, 1812". On June 26, 1812 Beethoven wrote out an affectionate dedicatory message to her daughter on his piano trio in B flat (WoO 39).

Antonie, the daughter of Johann-Melchior von Birkenstock, was married in 1798 to the Frankfurt banker Franz Brentano, who was a good friend of Beethoven during the family's short stay in Vienna (the purpose of which was to tie up the Birkenstock estate after the death of Antonie's father). His half-sister was Bettina von Arnim née Brentano, who may have introduced Antonie to the composer in 1811.