Działdowo d͡ʑau̯ˈdɔvɔ (Soldau) (Old Prussian: Saldawa) is a town in northern Poland with 20,935 inhabitants as of December 2021, the capital of Działdowo County. As part of Masuria, it is situated in the Warmian-Masurian Voivodeship (since 1999), Działdowo belonged previously to Ciechanów Voivodeship (1975–1998). The town is a major railroad junction connecting the capital city of Warsaw with Gdańsk and Olsztyn to the north.
The first settlement in the vicinity, known as Sasinowie in Polish and Sassen in German, was established by the Old Prussians, an indigenous Baltic tribe. The Teutonic Knights conquered the region and built a castle, a wing of which still remains. The new settlement near the castle founded by Mikołaj z Karbowa and named Soldov was granted town privileges on 14 August 1344 by the Grand Master Ludolf König. The name Dzialdoff was first written on a 1409 map during the Polish–Lithuanian–Teutonic War.
In 1444, the town joined the anti-Teutonic Prussian Confederation, at the request of which King Casimir IV Jagiellon signed the act of incorporation of the region to the Kingdom of Poland in 1454, and then the townspeople expelled the Teutonic Knights and recognized the Polish King as rightful ruler. During the subsequent Thirteen Years’ War, the town was briefly captured by the Teutonic Knights in 1455. After the peace treaty signed in Toruń in 1466, it became a part of Poland as a fief held by the State of the Teutonic Knights.
Within the Duchy of Prussia (a Polish fiefdom until 1657) the settlement converted to Lutheranism during the Protestant Reformation of the 16th century. The following communes belonged to the Protestant parish of Soldau: Amalienhof, Borowo, Bursch, Cämmersdorf, Gajowken, Hohendorf, Kyschienen, Königshagen, Kurkau, Niederhof, Pierlawken, Pruschinowo, and Rudolfsfelde.
In 1701 the town became part of the Kingdom of Prussia, from 1773 on within the newly formed province of East Prussia. Within the Kingdom of Prussia and the later German Empire, the settlement developed into an important Prussian Eastern Railway junction in the second half of the 19th century.