Mjölby is a city and the seat of Mjölby Municipality, Östergötland County, Sweden with 28,544 inhabitants in 2024. Mjölby is located by the rivulet Svartån. The name Mjölby is derived from Mölloby, which comes from mylna or mölna, meaning 'mill'. Due to the rapids of Svartån and the fertile soils of the surrounding plains, Mjölby is a natural place for a mill. During the Dacke War in 1542–1543, Mjölby was the northernmost point reached by the rebels from Småland.