Bad Oeynhausen (German: [baːt ˈʔøːnˌhaʊ̯zn̩] ⓘ) is a spa town on the southern edge of the Wiehengebirge in the district of Minden-Lübbecke in the East-Westphalia-Lippe region of North Rhine-Westphalia, Germany.

In the village of Bergkirchen, which belongs to Bad Oeynhausen, a wellspring sanctuary existed in pre-Christian (Saxon) times at the local crossing of the Wiehengebirge, which was replaced in the 9th century by a church.

In 753 Pepin the Short, according to the Royal Frankish Annals, stopped over ad locum qui dicitur Rimiae, so that Rehme is commonly accepted as the oldest part of town.

The origin myth of Bad Oeynhausen relates that in 1745 a local farmer named Sültemeyer noticed that after his pigs had wallowed in nearby mud they had a salty crust on their backs and he decided to investigate the source.

After public awareness of this finding, King Frederick II of Prussia ordered the construction of a saltworks, which was named "Royal Saline Neusalzwerk".

In 1848 King Frederick William IV of Prussia renamed it to "Royal Bath (German: Bad) Oeynhausen", and this name was retained after receiving its own town charter in 1860.

Among these the Jordansprudel, drilled in 1926, is best known and with a capacity of 6000 L/min and a total height of up to 40 m it is the world's highest carbonated thermal salt spring and de facto the town's landmark.

Under Nazi Germany, Bad Oeynhausen hosted a synod of the Confessing Church, as well as the home congregation to Jakob Emil Karl Koch, a leading member.