Jinju Fortress was not just another walled city. It was the strategic lock on the road to Jeolla Province, the rich southwestern region that Japan had not yet conquered. Japanese commanders Ukita Hideie and Hosokawa Tadaoki understood this clearly: taking Jinju would open a new front, allow them to crush guerrilla leader Gwak Jaeu's irregular forces operating in the area, and deliver the plunder of Korea's most productive agricultural region. They assembled an army of 30,000 soldiers drawn from multiple contingents and set out to take Changwon first, then Jinju. The Seventh Contingent arrived before the fortress walls on November 8, expecting the same pattern of collapse they had seen at other Korean strongholds throughout the invasion.

What they encountered instead was a garrison that fought back with unfamiliar ferocity. Kim Si-min had trained his men on the 170 arquebuses he had recently acquired, weapons equivalent to those the Japanese used. When the attackers charged with scaling ladders and siege towers, the Koreans answered with volleys of cannon fire, arrows, and arquebus rounds. Hosokawa tried shifting his approach, deploying his own arquebusiers to provide covering fire for troops climbing the walls. The Koreans ignored the incoming bullets and smashed the ladders with rocks and axes. Then they began lobbing mortars down onto the attackers massed at the wall's base. Japanese casualties mounted with every assault. The Taikoki, a Japanese chronicle, recorded the shock of meeting resistance this organized at a fortress the commanders had expected to take easily.

After three days of continuous fighting, a Japanese bullet struck Kim Si-min in the side of the head. He fell, unable to command. The Japanese pressed harder, hoping his loss would break Korean morale. It did not. The defenders fought on even as their ammunition ran low and their general lay dying. That night, Gwak Jaeu arrived with a small band of Righteous Army guerrillas, too few to relieve the garrison but enough to create a diversion. His men blew horns, beat drums, and made as much noise as a force many times their size. Behind them, some 3,000 additional irregulars materialized from the surrounding countryside. The Japanese commanders, suddenly unsure of how many Koreans they faced in the dark, made the calculation that continuing the siege was no longer worth the risk.

The Japanese withdrew. The road to Jeolla remained closed, and Korean morale soared. The first Siege of Jinju entered Korean memory alongside the Battle of Hansan Island and the Battle of Haengju as one of the three greatest victories of the Imjin War, a conflict that otherwise brought Korea devastating losses. Kim Si-min died of his wounds and became one of the war's most revered heroes. But the triumph was temporary. The Japanese returned the following summer with an even larger force, took the fortress in a second siege in 1593, and burned Jinju to the ground. The city was rebuilt. The memory of its defense was not so easily replaced; it became a story Koreans told about what stubbornness and ingenuity could accomplish against overwhelming odds.