thumb|Poland and Lithuania 1386–1434
thumb|right|[[Mound of the Horodło Union; the monument was built in 1861 in Horodło to celebrate the 448th anniversary of the union.]]
The Union of Horodło or Pact of Horodło was a set of three acts signed in the town of Horodło on 2 October 1413. The first act was signed by Władysław II Jagiełło, King of Poland, and Vytautas, Grand Duke of Lithuania. The second and third acts were composed by the Polish nobility (szlachta) and Lithuanian boyars, respectively. Selected Lithuanian nobles, 47 in total, were adopted into Polish heraldic families and granted Polish coats of arm. This symbolic gesture signified their desire to adopt Western customs and integrate into Western society.