Downtown Indianapolis is a neighborhood area in and the central business district of Indianapolis, Indiana, United States. Downtown is bordered by Interstate 65, Interstate 70, and the White River, and is situated near the geographic center of Marion County. Downtown emerged from the original 1821 town plat for Indianapolis—often referred to as the Mile Square—to encompass a broader geographic area of the central city, containing several smaller historic neighborhoods.

Downtown Indianapolis is the cultural, economic, and political center of the Indianapolis metropolitan area. Downtown anchors the city's burgeoning leisure and hospitality sector, home to nearly 8,000 hotel rooms and the city's major sports and convention facilities. Most of the city's monuments and memorials, performing arts venues, and museums are located downtown, as well as numerous parks, historic sites, and districts. Since 1825, the respective seats of municipal and state governments have operated from downtown Indianapolis.

When Indianapolis was founded in 1820, the new capital city was planned to occupy an area of one square mile (2.6 km²) near the confluence of Fall Creek and the White River, close to the geographic center of Marion County. The plat included eastern and western "anchors": the Indiana Statehouse (west), and the Indianapolis City Market and Marion County Courthouse (site of today's City-County Building), respectively. As the population increased and the city matured, the original plat continued to densify, developing into the region's central business district.

The city's historical core and contemporary central business district is sometimes referred to as the Mile Square. However, the downtown Indianapolis "neighborhood area" defines a broader geography of about 7 square miles (18 km²), incorporating the area bounded by 16th Street and Interstate 65 (north), Interstate 70 (south), Interstate 65/70 (east), and the White River (west). Downtown is generally bisected into four quadrants, divided by Meridian Street (north to south) and Washington Street (east to west).

Pogue's Run, a small creek, was channeled into a tunnel beneath downtown in the 1910s. It enters a culvert near New York and Dickson streets east of downtown, emptying into the White River near Kentucky and McCarty streets on downtown's southwest side.

Downtown Indianapolis contains 34 extant apartment buildings that are listed on the National Register of Historic Places in the Apartments and Flats of Downtown Indianapolis Thematic Resources. Entries in italics denote designated cultural districts.

Downtown Indianapolis dates to the city's founding as the state of Indiana's new capital in 1820 near the east bank of the White River. The state legislature appointed Alexander Ralston and Elias Pym Fordham to survey and design a town plan for Indianapolis, which was platted in 1821. Ralston's original plan for Indianapolis called for a town of one square mile (2.6 km²) bounded by North, East, South, and West streets (although they were not named at that time), with Governor's Circle, a large circular commons, at the center of town.

Ralston's grid pattern with wide roads and public squares extended outward from the four blocks adjacent to the Circle, and also included four diagonal streets, later renamed as avenues. Public squares were reserved for government and community use, but not all of these squares were used for this intended purpose. Ralston altered the grid pattern in the southeast quadrant to accommodate the flow of Pogue's Run, but a plat created in 1831 changed his original design and established a standard grid there as well.