The Siege of Tykocin occurred in 1655 when loyal Polish-Lithuanian Commonwealth forces besieged Tykocin, the castle of the treacherous Lithuanian grand hetman Janusz Radziwill.
On 17 August 1655, as King Charles X Gustav of Sweden invaded Poland, the Protestant Radziwill defected to the Swedes and declared the Grand Duchy of Lithuania a Swedish protectorate. On 20 October, Radziwill signed another treaty establishing a union between Sweden and Lithuania. However, Polish forces loyal to King Jan II Casimir held out against a Swedish siege of Jasna Gora, and the Tyszowce Confederation was formed to lead a national uprising against the Swedes and their collaborators. The sickly Radziwill soon found himself besieged in Tykocin by Michal Wolodyjowski's loyalist army, and, as Commonwealth forces stormed Tykocin, Radziwill's health gave way and he died on 31 December. Wolodyjowski, Jan Onufry Zagloba, and their companions barged into Radziwill's chamber shortly after his death, and they shed tears for their former lord. The desperate Swedish garrison of Tykocin later blew themselves up rather than be overwhelmed by the Poles.