Gen. Matsui Iwane (松井石根, 1878 - 1948) was the commanding officer of the Japanese expeditionary force responsible for the Nanjing Massacre in 1937.

He was born in Aichi prefecture. He attained the rank of general in the Japanese Army in 1933 and retired two years later, but came out of retirement to become the commander of the Japanese Shanghai Expeditionary Force during the Second Sino-Japanese War. He was personally appointed to that post by Emperor Hirohito on August 15, 1937, and while leaving the Imperial Palace in Tokyo, said to then-Prime Minister Konoe Fumimaro: "There's no solution except to break the power of Chiang Kai-shek by capturing Nanking. That is what I must do."

Matsui planned the attack of Nanjing for November of 1937. As he marched his troops westward from Shanghai, however, Hirohito was planning to place his cousin, Prince Asaka Yasuhiko, in command of the SEF. Asaka was promoted to Matsui's post on December 5, 1937, and Matsui became theater commander. On December 10, 1937, the SEF began its attack on Nanking, and the Kuomintang forces that remained surrendered on December 13, 1937. The Nanjing massacre began immediately afterwards, but halted only temporarily when Matsui marched triumphantly into Nanking on December 17, 1937.

While Matsui himself was not present during the beginning of the atrocities (he was ill at the time), he was aware of what his men were doing in the city, as were members of the Japanese foreign service who had followed the army into the city. Word began to trickle out of Nanking, and growing pressure was placed on the Imperial government to recall the SEF's officers.

Both Matsui and Asaka were recalled to Japan in 1938. Matsui retired, and returned to his hometown of Atami. Along with several others in the community, he built a large statue of Koa, the Goddess of Mercy, facing in the direction of Nanking.

He was decorated on April 29, 1940 for his role in the war.

In 1948, the International Military Tribunal for the Far East found him guilty of war crimes, and he was hanged that December at Sugamo Prison, alongside six others, including Tojo Hideki. He was 71 at the time of his death. In their decision, the Tribunal wrote:

The Tribunal is satisfied that Matsui knew what was happening. He did nothing, or nothing effective to abate these horrors. He did issue orders before the capture of the City enjoining propriety of conduct upon his troops and later he issued further orders to the same purport. These orders were of no effect as is now known, and as he must have known. It was pleaded in his behalf that at this time he was ill. His illness was not sufficient to prevent his conducting the military operations of his command nor to prevent his visiting the City for days while these atrocities were occurring. He was in command of the Army responsible for these happenings. He knew of them. He had the power, as he had the duty, to control his troops and to protect the unfortunate citizens of Nanking. He must be held criminally responsible for his failure to discharge this duty.

The first edition of The Rape of Nanking, by Iris Chang, followed the IMTFE's lead in blaming Matsui for the massacre, as Matsui planned the invasion of Nanking and was Asaka's commanding officer during the Rape. James Yin and Shi Young's book of the same title, however, blames Asaka for the massacre, and portrays Matsui as a helpless figurehead stuck between a prince and an emperor. The truth is a matter of continued debate.