At the Battle of Tsushima on 27 May 1905, Mikasa again led the 1st Fleet into combat, this time against the Second and Third Pacific Squadrons detached from the Baltic Fleet.
The ship opened fire at the battleship Knyaz Suvorov, the Russian flagship, at 14:10, and was joined by the battleship Asahi and the armoured cruiser Azuma shortly afterwards.
Within an hour the Japanese ships had started a serious fire aboard the Russian ship, badly wounded the fleet commander, Vice Admiral Zinovy Rozhestvensky, knocked out her rear twelve-inch gun turret, and jammed Knyaz Suvorov's steering so that she fell out of formation.
During this time, Mikasa was the focus of the Russian fire as the leading ship in the Japanese column and was hit by 6 twelve-inch and 19 six-inch shells.
They did very little damage and Tōgō was able to cross the T of the Russian squadrons.
Knyaz Suvorov's steering was later repaired, but she blundered between the Japanese and Russian fleets several times later in the battle and Mikasa fired three torpedoes at her to no avail.
Later in the battle, the ship appears to have fired mostly on the battleship Borodino although Fuji fired the shots that caused the Russian ship's magazines to explode and sink her.
At 18:04, a twelve-inch shell detonated prematurely in the barrel of the right gun of the forward turret, disabling the gun and knocking out the left gun until 18:40.
Another twelve-inch shell had exploded in that same barrel almost two hours earlier, but had not damaged the gun.
One six-inch gun jammed after firing 19 rounds, but the only other damage to any of the ship's guns was one six-inch gun that was disabled by a Russian shell of the same size that entered through the gunport.
She fired 124 twelve-inch shells during the battle, more than any other ship except Asahi's 142.
In total, Mikasa was hit more than 40 times during the battle, including 10 twelve-inch and 22 six-inch shells, but none of them seriously damaged her.
While Mikasa's casualties are not precisely known, the entire Japanese force combined only lost 110 men killed and 590 wounded to all causes during the battle.

Six days after the Treaty of Portsmouth that ended the war was signed, Mikasa sank at her moorings after a fire and magazine explosion at Sasebo on the night of 11/12 September 1905 that killed 251 crewmen.
She was refloated on 7 August 1906, reconstructed and repaired at Sasebo Naval Arsenal.
The navy took the opportunity to upgrade her existing armament with more powerful 45-calibre twelve-inch and six-inch guns during the two years it took to repair the ship.
Mikasa was restored to active service on 24 August 1908.
During World War I, she served on coast-defence duties, based at Maizuru, during 1914–15 and was then assigned to the Second and Fifth Squadrons, in that order, for the rest of the war.
The ship supported the Japanese intervention in Siberia during the Russian Civil War during 1921 and was reclassified on 1 September 1921 as a first-class coast-defence ship.
On 17 September, Mikasa ran aground near Askold Island off Vladivostok, but was not seriously damaged.
At the Battle of Tsushima on 27 May 1905, Mikasa again led the 1st Fleet into combat, this time against the Second and Third Pacific Squadrons detached from the Baltic Fleet. The ship opened fire at the battleship Knyaz Suvorov, the Russian flagship, at 14:10, and was joined by the battleship Asahi and the armoured cruiser Azuma shortly afterwards. Within an hour the Japanese ships had started a serious fire aboard the Russian ship, badly wounded the fleet commander, Vice Admiral Zinovy Rozhestvensky, knocked out her rear twelve-inch gun turret, and jammed Knyaz Suvorov's steering so that she fell out of formation. During this time, Mikasa was the focus of the Russian fire as the leading ship in the Japanese column and was hit by 6 twelve-inch and 19 six-inch shells. They did very little damage and Tōgō was able to cross the T of the Russian squadrons. Knyaz Suvorov's steering was later repaired, but she blundered between the Japanese and Russian fleets several times later in the battle and Mikasa fired three torpedoes at her to no avail. Later in the battle, the ship appears to have fired mostly on the battleship Borodino although Fuji fired the shots that caused the Russian ship's magazines to explode and sink her. At 18:04, a twelve-inch shell detonated prematurely in the barrel of the right gun of the forward turret, disabling the gun and knocking out the left gun until 18:40. Another twelve-inch shell had exploded in that same barrel almost two hours earlier, but had not damaged the gun. One six-inch gun jammed after firing 19 rounds, but the only other damage to any of the ship's guns was one six-inch gun that was disabled by a Russian shell of the same size that entered through the gunport. She fired 124 twelve-inch shells during the battle, more than any other ship except Asahi's 142. In total, Mikasa was hit more than 40 times during the battle, including 10 twelve-inch and 22 six-inch shells, but none of them seriously damaged her. While Mikasa's casualties are not precisely known, the entire Japanese force combined only lost 110 men killed and 590 wounded to all causes during the battle.
Six days after the Treaty of Portsmouth that ended the war was signed, Mikasa sank at her moorings after a fire and magazine explosion at Sasebo on the night of 11/12 September 1905 that killed 251 crewmen. She was refloated on 7 August 1906, reconstructed and repaired at Sasebo Naval Arsenal. The navy took the opportunity to upgrade her existing armament with more powerful 45-calibre twelve-inch and six-inch guns during the two years it took to repair the ship. Mikasa was restored to active service on 24 August 1908. During World War I, she served on coast-defence duties, based at Maizuru, during 1914–15 and was then assigned to the Second and Fifth Squadrons, in that order, for the rest of the war. The ship supported the Japanese intervention in Siberia during the Russian Civil War during 1921 and was reclassified on 1 September 1921 as a first-class coast-defence ship. On 17 September, Mikasa ran aground near Askold Island off Vladivostok, but was not seriously damaged.
Japanese battleship Mikasa