Mathilde Wesendonck(23 December 1828 - 31 August 1902) was a minor German poet, who is best known as the friend and possibly mistress of Richard Wagner, who set five songs to her words, the Wesendonck Lieder.

She married the silk merchant Otto Wesendonck (sometimes seen as von Wesendonk). Otto was a great admirer of Wagner’s music, and after he and Mathilde met the composer in Zurich in 1852, he placed a cottage on his estate at Wagner's disposal. By 1857, Wagner had become infatuated with Mathilde. It is not known whether she returned his affections to the same degree, or if the affair – such as it was - was ever consummated. Nevertheless, the affair inspired Wagner to put aside his work on Der Ring des Nibelungen (which would not be resumed for the next twelve years) and begin work on Tristan und Isolde.