Iwane Matsui (松井 石根, Matsui Iwane, July 27, 1878 – December 23, 1948) was a general in the Imperial Japanese Army and the commander of the expeditionary force sent to China in 1937. He was convicted of war crimes and executed by the Allies for his involvement in the Nanjing Massacre.

Born in Nagoya, Matsui chose a military career and served in combat during the Russo-Japanese War (1904–05). He volunteered for an overseas assignment there shortly after graduating from the Army War College in 1906. As Matsui rose through the ranks, he earned a reputation as the Japanese Army's foremost expert on China, and he was an ardent advocate of pan-Asianism. He played a key role in founding the influential Greater Asia Association.

Matsui retired from active duty in 1935 but was called back into service in August 1937 at the start of the Second Sino-Japanese War to lead the Japanese forces engaged in the Battle of Shanghai. After winning the battle Matsui succeeded in convincing Japan's high command to advance on the Chinese capital city of Nanjing. The troops under his command who captured Nanjing on December 13 were responsible for the notorious Nanjing Massacre.

Matsui finally retired from the army in 1938. Following Japan's defeat in World War II he was convicted of war crimes at the International Military Tribunal for the Far East (IMTFE) and executed by hanging. He and other convicted war criminals were enshrined at Yasukuni Shrine in 1978, an act that has stirred controversy.