The Siege of Sanadamaru occurred on 12/4 of Keichō 19 (1615) on the southern end of Ōsaka Castle in Settsu Province.  This battle was among those occurring during the Winter Campaign of the Siege of Ōsaka waged between the Toyotomi family and the Edo bakufu army.  The fighting raged at Sanadamaru in addition to along the entire southern side of Ōsaka Castle.  Sanadamaru was a smaller castle located to the south side of the Hirano entrance of Ōsaka Castle.

After the Toyotomi forces abandoned the surrounding fortresses and evacuated to Ōsaka Castle, the bakufu army surrounded them.  In front of Sanadamaru was a contingent of 12,000 soldiers led by Maeda Toshitsune along with several thousand others at the Hatchōme and Tanimachi entrances including Nanbu Toshinao, Matsukura Shigemasa, and Sakakibara Yasukatsu, in addition to 4,000 troops led by Ii Naotaka, 10,000 troops led by Matsudaira Tadanao, and several thousand others in formations.  Inside Sanadamaru, there were 5,000 troops under the command of Sanada Nobushige, along with Kimura Shigenari, Gotō Mototsugu, and Chōsokabe Morichika at the Hatchome and Tanimachi entrances with a total of over 12,000 troops deployed.

On 12/2 of Keichō 19 (1615), Tokugawa Ieyasu headed toward Chausuyamajin Castle (also known as Ōtsuka Castle) to conduct inspections and, after Tokugawa Hidetada also joined, returned to base.  Ieyasu instructed Maeda Toshitsune to dig trenches and build defensive mounds but not to attack the castle.  In front of Sanadamaru stood a hill called Sasayama where the Sanada positioned soldiers.  After the Maeda forces began to dig the trenches, the Sanada troops sniped at them with arquebus fire from atop Sasayama, impeding the progress of their work.

On 12/3, inside Ōsaka Castle, it was discovered that Nanjō Mototada was colluding with the bakufu army.  Mototada was forced to commit seppuku inside the castle, but the Toyotomi army pretended that the Nanjō continued to collude with them, deceiving the bakufu army.

On 1/3 of Keichō 20 (1616), the Maeda forces who were struggling with the Sanada forces firing at them from atop Sasayama made plans to capture the site.  Under cover of darkness, Honda Masashige, Yamazaki Naganori and others in the vanguard of the Maeda army climbed Sasayama to launch an attack, but the Sanada forces evacuated to the castle so the effort was in vain.  At daybreak, the Sanada forces incited the Maeda who responded by storming Sanadamaru.  Once the Maeda drew sufficiently close to the castle walls, the Sanada rained arquebus fire down on them.  Maeda Toshitsune became upset after the troops attacked without an order to do so and, as a result, incurred losses.  He then had his forces attempt a retreat.

Upon learning of the attack by the Maeda forces, the Ii and Matsudaira soldiers followed their lead by launching attacks against the Hatchōme and Tanimachi entrances.  At this time, the storehouse for gunpowder inside the castle accidently exploded.  After misinterpreting the explosion as an act of collusion by the Nanjō, the bakufu army intensified their attack.  The Toyotomi army responded by killing many of the besieging forces who rushed the walls of the castle.  After receiving reports of the calamitous assault, Ieyasu ordered a withdrawal.  The bakufu forces attacked without protection of bamboo and steel shields so could not easily move in response to the attacks from enemy forces.  The fighting finally wound down after 3:00 PM.

After withdrawing, Ieyasu moved his base to Chausuyama while Hidetada moved his base to Okayama.  Ieyasu summoned his guest commanders and criticized them for their careless actions, sternly warning them to always utilize bamboo and steel shields in the future.