The Union of Horodło was a set of acts introduced in the town of Horodło in 1413. It ammended the earlier Polish-Lithuanian Unions of Krewo and Wilno-Radom by further unification of the Polish-Lithuanian state.

According to the act of the union, the Grand Duchy of Lithuania was to retain a separate Grand Duke and its own parliament. At the same time both the Polish and Lithuanian Sejms were to discuss all the important matters jointly. Also, institutions of castellans and voivods were introduced in Lithuania and the Lithuanian and Ruthenian noblety were granted equal rights to those of the szlachta in Poland. This lead to a large number of Ruthenian and Lithuanian noble families being adopted by the Polish szlachta clans and introduction of coats of arms in Lithuania. This affected all 47 major families and clans, as well as several other families.

See also:

• Polish-Lithuanian Union
• Polish heraldry