Marinette County is a county in the U.S. state of Wisconsin. As of the 2020 census, the population was 41,872. Its county seat is Marinette. Marinette County is part of the Marinette, WI–MI Micropolitan Statistical Area.

According to the U.S. Census Bureau, the county has a total area of 1,550 square miles (4,000 km²), of which 1,399 square miles (3,620 km²) is land and 151 square miles (390 km²) (9.7%) is water. It is the third-largest county in Wisconsin by land area and fifth-largest by total area. Part of Marinette County borders Lake Michigan, and this area is home to endemic plants.

Crivitz Municipal Airport (FAA LID: 3D1) serves Marinette County and surrounding communities.

As of the 2020 census, the county had a population of 41,872 and a median age of 49.3 years. 19.0% of residents were under the age of 18 and 25.2% of residents were 65 years of age or older. For every 100 females there were 103.3 males, and for every 100 females age 18 and over there were 102.4 males age 18 and over.

The population density was 29.9 people per square mile (11.5 people/km²). There were 29,189 housing units at an average density of 20.9 units per square mile (8.1 units/km²).

The racial makeup of the county was 93.4% White, 0.5% Black or African American, 0.6% American Indian and Alaska Native, 0.4% Asian, <0.1% Native Hawaiian and Pacific Islander, 1.1% from some other race, and 4.0% from two or more races. Hispanic or Latino residents of any race comprised 2.6% of the population.

39.2% of residents lived in urban areas, while 60.8% lived in rural areas.

There were 18,735 households in the county, of which 22.1% had children under the age of 18 living in them. Of all households, 47.3% were married-couple households, 22.1% were households with a male householder and no spouse or partner present, and 22.4% were households with a female householder and no spouse or partner present. About 32.7% of all households were made up of individuals and 15.5% had someone living alone who was 65 years of age or older.