Piqua is located at 40°8′51″N 84°14′53″W / 40.1475°N 84.24806°W / 40.1475; -84.24806 (40.147474, -84.247968).

According to the United States Census Bureau, the city has a total area of 10.9 square miles (28.3 km²), of which, 10.7 square miles (27.7 km²) of it is land and 0.2 square miles (0.6 km²) of it (2.19%) is water.

The Great Miami River runs through Piqua. The area at the south end of town on the east side of the river is known as Shawnee.

As of the census of 2000, there were 20,738 people, 8,263 households, and 5,585 families residing in the city. The population density was 1,939.2 people per square mile (749.0/km²). There were 8,886 housing units at an average density of 830.9/sq mi (320.9/km²). The racial makeup of the city was 94.21% White, 3.38% African American, 0.33% Native American, 0.44% Asian, 0.27% from other races, and 1.37% from two or more races. Hispanic or Latino of any race were 0.74% of the population.

There were 8,263 households out of which 32.7% had children under the age of 18 living with them, 50.4% were married couples living together, 12.9% had a female householder with no husband present, and 32.4% were non-families. 27.3% of all households were made up of individuals and 11.4% had someone living alone who was 65 years of age or older. The average household size was 2.47 and the average family size was 2.99.

In the city the population was spread out with 26.5% under the age of 18, 9.0% from 18 to 24, 28.9% from 25 to 44, 21.3% from 45 to 64, and 14.3% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 91.4 males. For every 100 females age 18 and over, there were 85.7 males.

The median income for a household in the city was $35,681, and the median income for a family was $41,804. Males had a median income of $31,808 versus $22,241 for females. The per capita income for the city was $18,719. About 9.6% of families and 12.2% of the population were below the poverty line, including 18.0% of those under age 18 and 7.9% of those age 65 or over.

Although officially incorporated in 1807, Piqua's history dates back to a previous settlement in 1747 called Fort Pickawillany located a mile northeast of the present city at the Great Miami River's intersection with Loramie Creek. Piqua started out as two separate communities, Upper Piqua and Lower Piqua in 1780 becoming one community by 1800.

Rossville (since incorporated into Piqua) was the first local African-American settlement after the Randolph slaves were freed following the passing of John Randolph in 1833. The Rossville neighborhood and Randolph cemetery are located on the northeast side of the river off North County Road 25-A.

Piqua was home to the first municipally-operated nuclear power plant, in operation from 1962 to 1966 earning its short lived nickname "The Atomic City." This facility was a major demonstration failure and the Atomic Energy Commission bought out the contract with the City of Piqua, for an early contract termination. During this period a name brand automotive battery, the "Piqua Atomic Power Plant" was manufactured and marketed locally.