Ionia County (/aɪˈoʊnjə/ eye-OH-nyə) is a county located in the U.S. state of Michigan. As of the 2020 census, the population was 66,804. The county seat is Ionia. The Ionia County Courthouse was designed by Claire Allen, a prominent southern Michigan architect.
 Ionia County is part of the Grand Rapids-Kentwood metropolitan statistical area.
 The county is named for Ionia, a historic area of Greece.
 According to the US Census Bureau, the county has a total area of 580 square miles (1,500 km2), of which 571 square miles (1,480 km2) is land and 8.7 square miles (23 km2) (1.5%) is water.
 As of the 2000 census, there were 61,518 people, 20,606 households, and 15,145 families residing in the county. The population density was 107 inhabitants per square mile (41/km2). There were 22,006 housing units at an average density of 38 per square mile (15/km2). The racial makeup of the county was 91.96% White, 4.56% Black or African American, 0.56% Native American, 0.32% Asian, 0.01% Pacific Islander, 1.04% from other races, and 1.55% from two or more races. 2.78% of the population were Hispanic or Latino of any race. 29.0% were of German, 14.7% American, 11.6% English, 8.9% Irish and 5.3% Dutch ancestry. 96.3% spoke English and 2.6% Spanish as their first language.
 There were 20,606 households, out of which 38.10% had children under the age of 18 living with them, 58.70% were married couples living together, 10.10% had a female householder with no husband present, and 26.50% were non-families. 21.90% of all households were made up of individuals, and 8.90% had someone living alone who was 65 years of age or older. The average household size was 2.70 and the average family size was 3.15.
 The county's population spread included 26.90% under the age of 18, 11.50% from 18 to 24, 31.00% from 25 to 44, 20.50% from 45 to 64, and 10.00% who were 65 years of age or older. The median age was 33 years. For every 100 females, there were 115.20 males. For every 100 females age 18 and over, there were 120.40 males.
 The median income for a household in the county was $43,074, and the median income for a family was $49,797. Males had a median income of $36,995 versus $25,443 for females. The per capita income for the county was $17,451. About 6.80% of families and 8.70% of the population were below the poverty line, including 10.40% of those under age 18 and 8.10% of those age 65 or over.
 Ionia County has been reliably Republican from the beginning. Since 1884, the Republican Party nominee has carried the county vote in 82% of the elections (29 of 35 elections).
 The county government operates the jail, maintains rural roads, operates the major local courts, records deeds, mortgages, and vital records, administers public health regulations, and participates with the state in the provision of social services. The county board of commissioners controls the budget and has limited authority to make laws or ordinances. In Michigan, most local government functions—police and fire, building and zoning, tax assessment, street maintenance, etc.—are the responsibility of individual cities and townships.
 (information as of February 2023)
 Public school districts serving Ionia County include:
 42°56′N 85°04′W﻿ / ﻿42.94°N 85.07°W﻿ / 42.94; -85.07