Weishan County (simplified Chinese: 微山县; traditional Chinese: 微山縣; pinyin: Wēishān Xiàn) is a county of southwestern Shandong province, People's Republic of China, bordering Jiangsu province to the south. It is under the administration of Jining City, and much of its area is occupied by Nansi, or Weishan Lake. The population was 677,893 in 1999.