The settlement took its name from the neighboring village and present-day district of Gitter (first mentioned in 1347 as up dem solte to Gytere). There is much to suggest that Salzgitter had town rights in the Middle Ages, but there is no clear evidence of this in the sources.
After the end of the Hildesheim Collegiate Feud, Salzgitter became part of Brunswick in 1523. Salzgitter later belonged to the diocese of Hildesheim. When it was transferred to Prussia in 1803, city rights were confirmed, but were revoked again in 1815 when Salzgitter was incorporated into the Kingdom of Hanover. In 1830, a brine bath was established in Salzgitter.
With the transfer of the Kingdom of Hanover to Prussia (Province of Hanover) in 1866, Salzgitter became a Prussian municipality, which was again granted city rights in 1929. The settlement of Vorsalz (1926) and the Liebenhall salt works district (1928) had previously been incorporated. The town belonged to the district of Goslar and, in addition to the core town of Salzgitter, also included some small settlements that can no longer be defined today, such as Gittertor. The village of Kniestedt was incorporated in 1936.
Due to the extremely large iron ore deposits in Salzgitter, which were first mentioned as early as 1310, the National Socialists founded the Reichswerke-AG (Hermann Göring Werke) for ore mining and ironworks in 1937. In 1938, the neighboring community of Gitter was incorporated. In order for the plant to develop well economically, a uniform administrative structure was required for the entire area.
Therefore, in accordance with the 1941 ordinance on territorial adjustments in the area of the Hermann-Göring-Werke Salzgitter, a uniform urban district (independent town) was formed with effect from April 1, 1942. For this purpose, the city of Salzgitter and the municipalities of Beinum, Flachstöckheim, Groß-Mahner, Hohenrode, Ohlendorf and Ringelheim, which also belonged to the district of Goslar, as well as 21 municipalities belonging to the Brunswick district of Wolfenbüttel, were united to form the urban district of Watenstedt-Salzgitter.
In 1951, the town was renamed "Salzgitter" and the former district of Salzgitter was given the name Bad ("Salzgitter-Bad") because of the brine baths there. Today, the Thermalsolbad and the graduation pavilion in the rose garden are a sign of the rich brine.