The Dryden pulp mill, also known as the Reed Mill, is a paper and pulp mill in Dryden, Ontario. During the 1960s and 70s, mercury poisoning from the mill caused one of Canada's worst environmental disasters: Dryden Chemicals Ltd dumped mercury into the English-Wabigoon River, upstream of Grassy Narrows First Nation, poisoning the fish which were their staple food. Members of the Grassy Narrows and the Whitedog communities downstream from the mill suffered severe mercury poisoning. Additionally, the mill was the subject of a W5 investigative report, published in 2021, into claims that workers involved in the Recovery Boiler #4 (RB4) project were intentionally exposed to toxic fumes by their employer during its construction between 2002 and 2004. As of 2024, the RB4 project is also the subject of an ongoing occupational disease investigation by Occupational Health Clinics for Ontario Workers (OHCOW).

The first iteration of the mill was initially built by Charles and Grant Gordon in 1909 on the west side of the Wabigoon River From the early twentieth century to the twenty-first century, the forest industry has played a significant role in Dryden's economy.

In 2019, the Supreme Court of Canada ruled that Weyerhaeuser Company and Resolute Forest Products—previously known as Abitibi-Consolidated—are responsible for remediating the mercury contamination. The environmental poisoning continues to cause significant health problems for the First Nations communities downstream. The Government of Canada is building a $20 million clinic to treat the First Nations "suffering from mercury poisoning".

Since a US$520 million purchase of the mill by Domtar from Weyerhaeuser, in 2007 the mill has produced Northern Bleached Softwood Kraft pulp product.

The city of Dryden, which is in the Kenora District of Northwestern Ontario, Canada, on Wabigoon Lake—the headwater of the Wabigoon River near the Manitoba border, was ideally located for the pulp-and-paper industry with its abundant hydroelectricity supply from the Wabigoon River and a plentiful supply of wood. Dryden, as of 2016, had a population of 7,749. Dryden is the second-largest city in the Kenora District of Northwestern Ontario, Canada, located on Wabigoon Lake. It is the smallest community in the province of Ontario designated as a city.

The plant is located upstream of White Dog First Nation and Grassy Narrows First Nation.

In 1909, Charles and Grant Gordon began the construction of a paper mill on the west side of the Wabigoon River, where a paper mill is currently located. The mill's location has some advantages, because it has an abundant electricity supply from the river and a plentiful supply of wood. During the late 1890s, there were several sawmills operating in the Dryden, Ontario area. They primarily supplied the builders of the Canadian Pacific Railway(CPR) with railway timber, and powered the many steam boilers used for mining in that area. Since the early 1900s and the opening of the first sawmill in Dryden, the forest industry has played a significant role in the economy of the city.

In 1911, the rights of the timber lease were transferred from the Gordon brothers to the Dryden Timber and Power Company because the building they were constructing burned down in 1910, and they did not have means to complete their project.