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

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.

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.