The Japanese began their attack with a small diversionary force consisting of four companies of infantry and one of engineers attacking Chinese positions on the night of 28 July 1894 from the front, while the main force of nine companies of infantry, one of cavalry and a battalion of artillery successfully outflanked the Chinese defenses by crossing the Ansong River. The battle lasted from about 03:30 to 05:30 in the early morning of 28 July 1894. The defenders, after a sharp engagement, were unable to hold Seonghwan, and fled to Asan, ten miles to the southwest, leaving behind a considerable amount of weapons and stores.

The Japanese forces pursued the Chinese to the city of Asan, but the surprise defeat at Seonghwan had a strong impact on Chinese morale, and the Japanese took Asan with relatively little resistance by 15:00 on 29 July 1894. Surviving Chinese forces fled towards Pyongyang, which it reached after a heavy march of 26 days, detouring to avoid Japanese positions near Seoul. Chinese casualties included 500 killed and wounded against 34 killed and 54 wounded for the Japanese

The defeat of the Chinese forces at Asan broke the possibility of a Chinese encirclement of the Korean capital of Seoul. On July 30, after the Japanese victory at Seonghwan, General Oshima's troops entered Asan undisturbed, and within the following week cleared the area of the remaining Chinese forces. Consequently, the southern and central parts of Korea came under complete Japanese control. The victorious Japanese army returned to Seoul on 5 August 1894.

Clashes between Chinese and Japanese forces at Pungdo and Seonghwan caused an irreversible alteration to Sino-Japanese relations and meant that a state of war now existed between the two countries. Following the battle, formal declarations of war were officially issued by the Chinese government on July 31 and the Japanese government on August 1.