The Battle of Hulao (Chinese: 虎牢之戰) or Battle of Sishui (汜水之戰, Wade–Giles: Ssŭ Shui), on 28 May 621 was the main and final battle of the Luoyang–Hulao campaign between the rival Tang, Zheng, and Xia regimes during the transition from Sui to Tang. It was a decisive victory for the Tang prince Li Shimin, through which he was able to subdue two rival warlords, Dou Jiande who headed the Xia regime in Hebei, and Wang Shichong, the self-declared emperor of the Zheng dynasty. The battle was fought at the strategically important Hulao Pass, east of Luoyang.

Following victories in the west that had established his credentials as a general, in August 620 Li Shimin marched against Wang Shichong. Tang troops blockaded Wang in his capital of Luoyang, while seizing the rest of Henan province. After failing in his efforts to break through the Tang siege, and suffering from ever greater privations, Wang solicited help from Dou Jiande. In April 621, Dou Jiande led a 100,000–120,000 strong army west to confront the Tang. Li Shimin's generals urged him to retreat west and protect the Tang core territory at Shanxi, but doing so would surrender the northeastern plains, at that time the heartland of China, to Dou. Consequently, Li Shimin took a gamble by leading a small force east to occupy the strategic Hulao Pass, while the bulk of his army was left behind continuing the siege of Luoyang. Ensconced in favourable defensive positions, the Tang managed to hold up the Xia advance. A large and heterogeneous army, the Xia lacked the flexibility to either outflank Li Shimin's position or abandon the Luoyang campaign and attack the exposed Tang heartland at Shanxi. As a result, the standoff between the two armies continued for several weeks.

Finally, when he judged the situation ripe, Li Shimin feigned detaching part of his forces north to entice an attack. When Dou took the bait and advanced to the Tang positions in battle order, Li Shimin held back his own troops for several hours, until the troops of Dou Jiande were exhausted from being made to wait in formation under the sun for the entire morning. Once signs of disorder began appearing among them, Li Shimin attacked, breaking the opposing army and capturing Dou Jiande. Subsequently, Wang Shichong, left with no other choice, surrendered Luoyang. Both his and Dou Jiande's states were absorbed by the Tang. Dou Jiande was later executed, resulting in some of his followers, led by Liu Heita, raising an unsuccessful rebellion against the Tang. Hulao marked the decisive turning point in the civil wars that followed the collapse of the Sui Dynasty, after which the eventual victory of the Tang was never in doubt.