Malignant pleural mesothelioma (MPM) is a highly aggressive tumor that arises from the surface of the pleura and is associated with a history of asbestos exposure. The tumor is characterized by a strong local invasiveness and a poor response to any single modality therapy. Therefore clinical outcome of patients with MPM is poor and median survival time of untreated patients with MPM is 7 months from initial diagnosis. The Wilms Tumor Protein 1 (WT1) is a transcription factor which is highly expressed by MPM and is involved in cellular development and survival. We evaluated the role of WT1 in two human MPM cell lines (MSTO and H2052) expressing high levels of WT1. We performed a knockdown of WT1 using siRNA. Knockdown of WT1 was confirmed by Westernblotting. After knockdown of WT1 we investigated the effect on proliferation, chemoresistance, chemotaxis and migration. We could demonstrate that knockdown of WT1 suppresses chemoresistance in both cell lines compared with control (scrambled siRNA). Additionally, WT1 knockdown reduces proliferation, chemotaxis and invasiveness of mesothelioma cell lines. WT1 reduces malignancy of malignant mesothelioma cell lines and might be a new molecular target in mesothelioma therapy. Further investigations are needed to discover the mechanisms of chemoresistance depending on WT1.