Matilda of Brandenburg (also called Mechthild; c. 1210 – 10 June 1261), a member of the House of Ascania, was first Duchess consort of Brunswick-Lüneburg from 1235 to 1252 by her marriage with the Welf duke Otto the Child.

Albert's uncle Count Bernhard of Anhalt had received the Duchy of Saxony after the deposition of the Welf duke Henry the Lion in 1180 and Matilda's father, ruling the Margraviate of Brandenburg since 1205, initially had been a loyal supporter of the Imperial Hohenstaufen dynasty.

Upon his death in 1220, he was succeeded by Matilda's brothers, John and Otto III, who continued to support the Saxon Welfs in the struggle for their allodial lands around Brunswick.

At the same time, Matilda's younger sister Elisabeth married the Ludovingian landgrave Henry Raspe of Thuringia.

Duke Otto died in 1252 and was succeeded by his eldest son Albert, who nevertheless in 1267 had to divide his Brunswick heritage with his younger brother John.