The Battle of Wiłkomierz took place on September 1, 1435, near Ukmergė in the Grand Duchy of Lithuania. With the help of military units from the Kingdom of Poland, the forces of Grand Duke Sigismund Kęstutaitis soundly defeated Švitrigaila and his Livonian allies. The battle was a decisive engagement of the Lithuanian Civil War (1432–1438). Švitrigaila lost most of his supporters and withdrew to southern Grand Duchy; he was slowly pushed out and eventually made peace. The damage inflicted upon the Livonian Order has been compared to the damage of Battle of Grunwald upon the Teutonic Order. It was fundamentally weakened and ceased to play a major role in Lithuanian affairs. The battle can be seen as the final engagement of the Lithuanian Crusade.

The battle reduced the power of the Livonian Order as its army was defeated, its Grand Master killed and of its many senior officers killed or taken prisoner. The damage to the Livonian Order caused by the battle is often compared to the consequences that the Battle of Grunwald (1410) had on the Teutonic Knights. The peace treaty was signed on December 31, 1435 in Brześć Kujawski. The Teutonic and Livonian Orders promised not to interfere in internal matters of Lithuania or Poland. According to the treaty, even the Pope or the Holy Roman Emperor could not force the Orders to violate the accords. That provision was meant particularly against Sigismund, Holy Roman Emperor, and his attempts in playing Poland and Teutonic Order against each other.