Kasuga and Nisshin reached Yokosuka on 16 February just as Japan initiated hostilities with its surprise attack on Port Arthur, and began working up with Japanese crews.
The sisters were assigned to reinforce the battleships of the 1st Division of the 1st Fleet under the overall command of Admiral Tōgō Heihachirō on 11 April.
In an effort to block the Russian ships in Port Arthur, Togo ordered a minefield laid at the mouth of the harbor on 12 April and Kasuga and Nisshin were tasked to show themselves "as a demonstration of our power".
Tōgō successfully lured out a portion of the Russian Pacific Squadron, including Vice Admiral Stepan Makarov's flagship, the battleship Petropavlovsk.
When Makarov spotted the five Japanese battleships and Kasuga and Nisshin, he turned back for Port Arthur and his flagship ran into the minefield just laid by the Japanese.
The ship sank in less than two minutes after one of her magazines exploded, and Makarov was one of the 677 killed.
In addition to this loss, the battleship Pobeda was damaged by a mine.
Emboldened by his success, Tōgō resumed long-range bombardment missions, making use of the long-range capabilities of Kasuga and Nisshin's guns to blindly bombard Port Arthur on 15 April from Pigeon Bay, on the southwest side of the Liaodong Peninsula, at a range of 9.5 kilometers (5.9 mi).
In early May, the sisters fired at ranges up to 18 kilometers (11 mi) although this proved to be ineffective.

On 15 May, the battleships Yashima and Hatsuse were sunk by Russian mines.
That same day, off Port Arthur, Kasuga collided in the fog with the protected cruiser Yoshino, which capsized and sank with the loss of 318 officers and enlisted men.
With a third of Japan's battleships lost, Tōgō decided to use Kasuga and Nisshin in the line of battle together with his four remaining battleships.
The first test of this decision would have occurred on 23 June when the Pacific Squadron sortied in an abortive attempt to reach Vladivostok, but the new squadron commander, Rear Admiral Wilgelm Vitgeft, ordered the squadron to return to Port Arthur when it encountered the Japanese battleline (including Kasuga and Nisshin) shortly before sunset, as he did not wish to engage his numerically superior opponents in a night battle.
On 27 July, the sisters forced a Russian force of one battleship and several cruisers and gunboats to return to port because of long-range gun fire after they sortied to provide fire support to the Russian Army.

Kasuga and Nisshin participated in the Battle of the Yellow Sea on 10 August, but only played a minor role as they were in the rear of the Japanese battleline.
During the battle, the ship's executive officer was Kantarō Suzuki, later Prime Minister of Japan.
Kasuga was not significantly damaged, although she was hit three times with 11 crewmen wounded.
Kasuga fired 33 ten-inch shells along with an unknown number of eight-inch shells during the battle.
After the battle the sisters returned to Pigeon Bay where they engaged the Russian fortifications.

At the Battle of Tsushima on 26 May 1905, Kasuga was fifth in the line of battle.
At about 14:10, Kasuga opened fire on the battleship Oslyabya, the lead ship in the second column of the Russian fleet.
Due the limited visibility and heavy smoke during the battle detailed knowledge is not available about her activities during the rest of the day's action.
The surviving Russian ships had been located near the Liancourt Rocks by the Japanese the following morning and Tōgō reached them about 10:00.
Heavily outnumbering the Russians, he opted for a long-range engagement to minimize any losses and Kasuga opened fire at the obsolete battleship Imperator Nikolai I at a range of 9,100 meters (10,000 yd).
The ship hit her target's funnel on her third salvo and the Russians surrendered shortly afterwards.

During the course of the battle, Kasuga fired 50 ten-inch and 103 eight-inch shells; due to the poor visibility and sinking of many Russian ships, the only confirmed hits made by the ship were two against the battleship Oryol with ten-inch shells,
Kasuga and Nisshin reached Yokosuka on 16 February just as Japan initiated hostilities with its surprise attack on Port Arthur, and began working up with Japanese crews. The sisters were assigned to reinforce the battleships of the 1st Division of the 1st Fleet under the overall command of Admiral Tōgō Heihachirō on 11 April. In an effort to block the Russian ships in Port Arthur, Togo ordered a minefield laid at the mouth of the harbor on 12 April and Kasuga and Nisshin were tasked to show themselves "as a demonstration of our power". Tōgō successfully lured out a portion of the Russian Pacific Squadron, including Vice Admiral Stepan Makarov's flagship, the battleship Petropavlovsk. When Makarov spotted the five Japanese battleships and Kasuga and Nisshin, he turned back for Port Arthur and his flagship ran into the minefield just laid by the Japanese. The ship sank in less than two minutes after one of her magazines exploded, and Makarov was one of the 677 killed. In addition to this loss, the battleship Pobeda was damaged by a mine. Emboldened by his success, Tōgō resumed long-range bombardment missions, making use of the long-range capabilities of Kasuga and Nisshin's guns to blindly bombard Port Arthur on 15 April from Pigeon Bay, on the southwest side of the Liaodong Peninsula, at a range of 9.5 kilometers (5.9 mi). In early May, the sisters fired at ranges up to 18 kilometers (11 mi) although this proved to be ineffective.
On 15 May, the battleships Yashima and Hatsuse were sunk by Russian mines. That same day, off Port Arthur, Kasuga collided in the fog with the protected cruiser Yoshino, which capsized and sank with the loss of 318 officers and enlisted men. With a third of Japan's battleships lost, Tōgō decided to use Kasuga and Nisshin in the line of battle together with his four remaining battleships. The first test of this decision would have occurred on 23 June when the Pacific Squadron sortied in an abortive attempt to reach Vladivostok, but the new squadron commander, Rear Admiral Wilgelm Vitgeft, ordered the squadron to return to Port Arthur when it encountered the Japanese battleline (including Kasuga and Nisshin) shortly before sunset, as he did not wish to engage his numerically superior opponents in a night battle. On 27 July, the sisters forced a Russian force of one battleship and several cruisers and gunboats to return to port because of long-range gun fire after they sortied to provide fire support to the Russian Army.
Kasuga and Nisshin participated in the Battle of the Yellow Sea on 10 August, but only played a minor role as they were in the rear of the Japanese battleline. During the battle, the ship's executive officer was Kantarō Suzuki, later Prime Minister of Japan. Kasuga was not significantly damaged, although she was hit three times with 11 crewmen wounded. Kasuga fired 33 ten-inch shells along with an unknown number of eight-inch shells during the battle. After the battle the sisters returned to Pigeon Bay where they engaged the Russian fortifications.
At the Battle of Tsushima on 26 May 1905, Kasuga was fifth in the line of battle. At about 14:10, Kasuga opened fire on the battleship Oslyabya, the lead ship in the second column of the Russian fleet. Due the limited visibility and heavy smoke during the battle detailed knowledge is not available about her activities during the rest of the day's action. The surviving Russian ships had been located near the Liancourt Rocks by the Japanese the following morning and Tōgō reached them about 10:00. Heavily outnumbering the Russians, he opted for a long-range engagement to minimize any losses and Kasuga opened fire at the obsolete battleship Imperator Nikolai I at a range of 9,100 meters (10,000 yd). The ship hit her target's funnel on her third salvo and the Russians surrendered shortly afterwards.
During the course of the battle, Kasuga fired 50 ten-inch and 103 eight-inch shells; due to the poor visibility and sinking of many Russian ships, the only confirmed hits made by the ship were two against the battleship Oryol with ten-inch shells,
Japanese cruiser Kasuga