The most common website for flat offers and requests; also available in English, highly recommended:


Made by student government (AStA) of TU Dortmund and FH Dortmund; only in German and with a significantly lower number of flat offers:


There are also some facebook groups like...

WG & Wohnung Dortmund

TU Dortmund WG Börse


Also possible, but not recommended for short-term stay: (watch out, they often charge you extra commission!)