The town lies at the crossroads of important railroads and highways linking it to Cluj-Napoca, Baia Mare, Satu Mare, Deda, Bistriţa, and Vatra Dornei.
Dej is an old salt mining town built on a hilly ground. The first documents attesting the existence of this city go back to 1061 and 1214. Massive salt reserves were found in the area in Roman times. The Dej fortress was built sometime between 1214 and 1235.
King Andrew II of Hungary raised Dés to the privileged status of a free royal town. In 1241 the city was invaded by Tatars. The old mines were exhausted by 1717; the new mines are still in operation today. Some of the galleries of the salt mine are believed to be more than 15 kilometers long. The population of Dej used to consist mostly of Transylvanian Saxons, who settled here from Germany; their number decreased over centuries.
In 1638, Dej was the site for the show trial staged against the members of the Sabbatarians (Hungarian: Szombatosok), a sect formed during the Protestant movement; they were sentenced to death. The execution took place in Beszterce (Bistriţa).
In 1717, an attack by the Tatars of Crimea struck Dej.
From 1876 to 1920 Dej was capital of the Szolnok-Doboka County in the Kingdom of Hungary; from 1920 to 1940, capital of the Someş County in Romania.
The town's landmark is the Hungarian Reformed Church, built in the second half of the 15th century. The church displays Gothic elements carved in stone. The tower is 72 meters high, and the fortifying walls were erected in the 16th century, then torn down during a renovation in the 1880s. There is also a monastery of the Franciscan order in Dej. Dej also has a large synagogue located close to the Reformed Church.
In 1944 the Jews of Dej, along with the Jews from the surrounding areas, were rounded up and marched to the nearby bungar forest. Some 8000 Jews were left exposed to the elements for approximately one month. In June 1944 in a series of trains the Jews were deported to Auschwitz for extermination. In 1944 the Jewish population accounted for approximately 25% of the entire city. In 2008, there are five Jews living in Dej. In front of the synagogue there is a memorial to the Jewish victims of the fascists.
Other sites of interest in Dej: "Dr. Teodor Mihaly" and "Dr. Alexandru Vaida-Voevod" memorial houses and the Ocna Dej salt mine, said to be suited for the treatment of locomotor system diseases, asthenia, debility, and rachitis.