Otsego County (/ɒtˈsiːɡoʊ/ ot-SEE-goh), formerly known as Okkuddo County, is a county located in the U.S. state of Michigan. As of the 2020 census, the population was 25,091. The county seat is Gaylord. The county was founded in 1840 and organized in 1875.

Otsego may be a Native American name meaning "place of the rock". However, an alternative theory is that it derives from a lake and a county in New York state, which are said to bear the name derived from a Mohawk Iroquoian word meaning either "clear water" or "meeting place." It may be a neologism coined by Henry Schoolcraft, who was a borrower of words and pieces of words from many languages (including Arabic, Greek, Latin, and various American Indian languages). See List of Michigan county name etymologies.

The county was created in 1840 as Okkuddo County (meaning "sickly water," although the reason for using a name with such a negative meaning is lost). The name was changed to Otsego in 1843, possibly named after the county in New York, with the name ultimately deriving from a Mohawk or Oneida word meaning "place of the rock." It was organized in 1875. On May 20, 2022, an EF3 tornado struck the county seat of Gaylord killing 2 and injuring 44 while causing major damage to the downtown business district and severely damaging a mobile home park.

According to the United States Census Bureau, the county has a total area of 526 square miles (1,360 km²), of which 515 square miles (1,330 km²) is land and 11 square miles (28 km²) (2.1%) is water. It is the fifth-smallest county by total area in Michigan. Although it is located on Michigan's Lower Peninsula, Otsego County is considered to be part of Northern Michigan.

Otsego County has more than 370 lakes, mostly in the southern part of the county. Otsego Lake is the county's largest and has a surface area of 1,972 acres (7.98 km²). Other large lakes in the southern part of the county include Big Lake, Big Bear Lake, Buhl Lake, Crapo Lake, Dixon Lake, Douglas Lake, Guthrie Lake, Heart Lake, Lake Tecon, Manuka Lake, Opal Lake, Pencil Lake, and Turtle Lake. The larger lakes in the northern part of the county are Five Lakes, Hardwood Lake, Lake Twenty Seven, and Pickerel Lake. Many of these are so-called 'kettle lakes,' formed by the melting of blocks of glacial ice, left as the glacier retreated, which created a depression in the soil.

Glaciers shaped the area, creating a unique regional ecosystem. A large portion of the area is the Grayling outwash plain, a broad outwash plain including sandy ice-disintegration ridges; jack pine barrens, some white pine-red pine forest, and northern hardwood forest. Large lakes were created by glacial action.

Headwaters of the Au Sable, Black, Manistee, Pigeon, and Sturgeon Rivers are in Otsego County. The Au Sable River watershed is the county's largest watershed.

As of the 2020 census, the county had a population of 25,091. The median age was 45.3 years. 20.7% of residents were under the age of 18 and 21.9% of residents were 65 years of age or older. For every 100 females there were 100.1 males, and for every 100 females age 18 and over there were 98.3 males age 18 and over.