The Battle of Saule (German: Schlacht von Schaulen; Latvian: Saules kauja; Lithuanian: Saulės mūšis or Šiaulių mūšis) was fought on September 22, 1236 between the Livonian Brothers of the Sword and pagan Samogitians. Between 48 and 60 knights were killed, including the Livonian Master, Volkwin. It was the earliest large-scale defeat suffered by the orders in Baltic lands.

The Sword-Brothers, the first Catholic military order established in the Baltic lands, was soundly defeated and its remnants accepted incorporation into the Teutonic Order in 1237. The battle inspired rebellions among the Curonians, Semigallians, Selonians, Oeselians, tribes previously conquered by the Sword-Brothers. Some thirty years' worth of conquests on the left bank of Daugava were lost.

To commemorate the battle, in 2000 the Lithuanian and Latvian parliaments declared September 22 to be the Day of Baltic Unity.

The Sword-Brothers were established in 1202 in Riga to conquer and convert pagan Baltic tribes to Christianity. By the 1230s under the leadership of Master Volkwin, the Order was coping with strained financial resources, decreasing manpower, and ill reputation. The Order was in conflict with the papacy under Pope Gregory IX and the Holy Roman Emperor, two of its biggest supporters, over Estonia.

However, on February 19, 1236, Pope Gregory IX issued a papal bull declaring a crusade against Lithuania. He targeted Samogitia, planning to conquer the coast of the Baltic Sea and connect with the Teutonic Knights in Prussia. The Sword-Brothers wanted to keep expanding along the Daugava River and was somewhat reluctant to march against Samogitia. In fall of 1236 a party of crusaders arrived from Holstein; it demanded to be led into a battle. Volkwin gathered a large war party, which included troops from Pskov Republic, Livonians, Latgallians, Estonians.

The knights marched southward into Samogitia, raiding and plundering local settlements. The locals had only a few days to gather troops for defense. On the knights' northward return, however, they encountered a determined group of Samogitians at a river crossing. Unwilling to risk losing their horses in the swampland, the Holsteiners refused to fight on foot, forcing the knights to camp for the night.

The next morning, on the day of Saint Maurice, the main pagan forces, likely led by Duke Vykintas, arrived at the camp. The Lithuanian light cavalry flung javelins at short range, which were highly effective against the unwieldy Livonian heavy cavalry. The swampy terrain was advantageous for lightly armed pagans. The slaughter of the Christian troops, including Volkwin, sowed the seeds of confusion in the Livonian ranks. The lightly armed native forces under the command of the Brothers soon fled from the battle.

Those crusaders and knights who tried to flee to Riga were allegedly killed by the Semigallians.