St. Stephen is a Canadian town in Charlotte County, New Brunswick, situated on the east bank of the St. Croix River around the intersection of New Brunswick Route 170 and the southern terminus of New Brunswick Route 3. The St. Croix River marks a section of the Canada–United States border, forming a natural border between Calais, Maine and St. Stephen. U.S. Route 1 parallels the St. Croix river for a few miles, and is accessed from St. Stephen by three cross-border bridges.

On 1 January 2023, St. Stephen annexed all or part of seven local service districts and was renamed the Municipal District of St. Stephen, retaining town status. Revised census figures have not been released.

The Peskotomuhkati people (formerly referred to as the Passamaquoddy) were the first to make their home along the St. Croix River. They dispersed and hunted inland in the winter; in the summer, they gathered more closely together on the coast and islands, and primarily harvested seafood, including porpoise. In 1604, French explorer Samuel de Champlain and his men spent a winter here. The Peskotomuhkati were forced off their original lands repeatedly by European settlers since that time.

During Queen Anne's War, in response to the French Raid on Deerfield, New Englander Major Benjamin Church raided the Acadian villages of Castine, Maine (then known as Penobscot). From the Raid on Castine, Church learned that Michel Chartier was granted the land of present-day St. Stephen and was building a fort at Passamaquoddy Bay. Church and his men arrived at the Passamaquoddy Bay on board the Province Galley, Gosport, Fearly, and several other vessels. Church travelled up the St. Croix River to St. Stephen and, on 7 June 1704, took Chartier by surprise and his family fled into the woods. On June 13, Church reported they were destroying the crops of the Acadians and the Acadians and Peskotomuhkati fired upon Church's troops, resulting in a three-hour exchange. Church killed and imprisoned 35 Acadians and Peskotomuhkati, one of Church's men was wounded, and the community was pillaged and plundered.

After the Raid on St. Stephen, Church moved on to raid other Acadian villages in the Raid on Grand Pré, the Raid on Piziquid, and the Raid on Chignecto.

St. Stephen was officially incorporated as a town in 1871.

Immediately prior to Confederation, the Legislature of New Brunswick passed a bill that incorporated the Houlton Branch Railway Company (HBRC) with the aim to build a railway between Debec, New Brunswick and Houlton, Maine. This was in hopes of completing a portion of the St. Andrews and Quebec Railway. In 1870, the Legislature of New Brunswick passed an Act, which authorized Charlotte County to issue debentures to pay a bonus of $15,000 to the HBRC in order to encourage it to complete the railway. The debenture could only be issued if authorized by a public meeting of the ratepayers of St. Stephen, who would assume the obligation of paying for the interest and principal of the debenture. A majority of the ratepayers of St. Stephen duly passed the necessary vote and the County issued the debenture. Some residents of St. Stephen who opposed the measure challenged the tax assessment in the Supreme Court of New Brunswick, arguing that the provincial Legislature lacked the constitutional authority to authorise a tax to support the building of an international railway, as that would intrude on the exclusive legislative authority of the Parliament of Canada.

In 1873, the Supreme Court of New Brunswick ruled that the provincial taxation statute was unconstitutional, because it intruded on federal jurisdiction over inter-jurisdictional railways. The supporters of the railway measure appealed the case to the Judicial Committee of the Privy Council, at that time the court of last resort for Canada within the British Empire. The Judicial Committee, in the case of Dow v Black, allowed the appeal and held that the taxation statute was within provincial authority.