Mechtild of Nassau, german Mechthild von Nassau, (before 1280 – 19 June 1323) was the youngest child of Adolf, King of the Romans and his wife Imagina of Isenburg-Limburg. Mechtild is also known as Matilda of Nassau. She was Duchess consort of Bavaria, by her marriage to Rudolf I, Duke of Upper Bavaria.
Mechtild was the youngest of eight children, however only Mechtild and three other siblings lived to adulthood: Gerlach I of Nassau-Wiesbaden, Walram III of Nassau-Wiesbaden and Adelheid, Abbess of Klarenthal Abbey.
Rudolf succeeded his father in 1294 and supported his father-in-law king Adolf of Nassau-Weilburg against his uncle, the Habsburg Albert of Austria. After Adolf’s death Rudolf joined Albert’s party but the strong dynastic policy of the new king caused a new conflict. Rudolf's mother, Matilda of Habsburg acted as regent for her son Rudolf as well as her other son, the future Louis IV, Holy Roman Emperor. A civil war against Louis due to new disputes on the partition of Bavaria was ended in 1313, when peace was made at Munich. Rudolf died in 1319, in England.
Louis IV, Holy Roman Emperor had taken the Palatinate by force of arms. In August 1322, the war finally came to an end, but only after Mechtild's death in June 1323, when the three brothers finally were able to make peace with their uncle. Louis' sons inherited Bavaria and Rudolf and Mechtild's sons inherited the Upper Palatinate and the Palatinate in line with the Treaty of Pavia (1329).
Metchild died 19 June 1323 in Heidelberg. She was buried at Klarenthal Abbey.
She married in Nuremberg 1 September 1294 Rudolf I, Duke of Bavaria. John I of Isenburg-Limburg helped make the final agreement. Mechtild and Rudolf had: