The Siege of Gassantoda Castle (月山富田城の戦い, Gassan Toda-jō no Tatakai) was a battle during the Sengoku period (16th century) of Japan.

Siege of Gassantoda Castle (1542)
Sannomaru walls
Date 1542–1543 Location Gassantoda Castle, Izumo Province 35°21′49″N 133°10′58″E / 35.36361°N 133.18278°E / 35.36361; 133.18278 Result Amago clan victory
Belligerents
Amago clan	Ōuchi clan Mōri clan
Commanders and leaders
Amago Haruhisa	Ōuchi Yoshitaka Mori Motonari Hironaka Takakane
Siege of Gassantoda Castle (Japan) Show map of Japan

The siege of the castle was personally led by Ōuchi Yoshitaka against Gassantoda Castle located within Izumo Province, under the control of Amago Haruhisa.

In this battle Mori Motonari penetrated deep into the Amago clan territory, but their supply line was broken and Kikkawa Okitsune (吉川興経) betrayed them. Motonari surrounded Gassantoda castle (富田城) but the Ōuchi troops retreated. During the retreat Motonari almost lost his life, but his general, Watanabe Hajime tried to sacrifice his life, so he can get away by fighting to the death. Motonari returned safely to Aki Province.

After a long, hard-fought siege, Haruhisa ended in victory, and as a result of the battle, the power of the Ōuchi clan weakened.

Yoshitaka, who had failed in his attempt, withdrew to his headquarter Yamaguchi, in which he indulged himself with "more and more in pleasures", until he was deposed by his retainer, Sue Harukata, in the Tainei-ji incident.

Siege of Gassantoda Castle (1565)
Karamente Walls
Date 1565–1566 Location Gassantoda Castle, Izumo Province Result Mōri clan victory
Belligerents
Amago clan	Mōri clan
Commanders and leaders
Amano Yoshihisa Amago Tomohisa Uyama Hisakane	Mōri Motonari Kikkawa Motoharu Kobayakawa Takakage

Siege of Gassantoda Castle (1569)
Sugatani Gate
Date 1569 Location Gassantoda Castle, Izumo Province Result Mōri clan victory
Belligerents
Mōri clan	Amago clan
Commanders and leaders
Amano Takashige	Amago Katsuhisa Yamanaka Yukimori Tachihara Hisatsuna