According to the U.S. Census Bureau, the county has a total area of 191 square miles (490 km²), of which 184 square miles (480 km²) is land and 6.8 square miles (18 km²) (3.5%) is water.
It is the third-smallest county in Pennsylvania by area.

Delaware County is roughly diamond- or kite-shaped, with the four sides formed by the Chester County boundary to the northwest, the boundary with the state of Delaware (a portion of the "Twelve Mile Circle") to the southwest, the Delaware River (forming the border with the state of New Jersey) to the southeast, and the city of Philadelphia and Montgomery County to the east and northeast.

The lowest point in the state of Pennsylvania is located on the Delaware River in Marcus Hook in Delaware County, where it flows out of Pennsylvania and into Delaware.
 The highest point in Delaware County is 500 feet at two points southeast of Wyola in Newtown Township .

Waterways in Delaware County generally flow in a southward direction and ultimately drain into the Delaware River.
 The waterways are, from west to east: the Brandywine River (forming a portion of the county's western boundary with Chester County), Naaman's Creek, Stoney Creek, Chester Creek, Ridley Creek, Crum Creek, Muckinipates Creek, Darby Creek and Cobbs Creek (forming a portion of the county's eastern boundary with Philadelphia).
 Crum Creek was dammed in 1931 near Pennsylvania Route 252 to fill Springton Lake (also known as Geist Reservoir), an approximately 391-acre (1.58 km²) drinking water reservoir maintained by Aqua America, the county's largest lake.

The Trainer Refinery and the Port of Chester are located along the shores of the Delaware River.
According to the U.S. Census Bureau, the county has a total area of 191 square miles (490 km²), of which 184 square miles (480 km²) is land and 6.8 square miles (18 km²) (3.5%) is water. It is the third-smallest county in Pennsylvania by area.
Delaware County is roughly diamond- or kite-shaped, with the four sides formed by the Chester County boundary to the northwest, the boundary with the state of Delaware (a portion of the "Twelve Mile Circle") to the southwest, the Delaware River (forming the border with the state of New Jersey) to the southeast, and the city of Philadelphia and Montgomery County to the east and northeast.
The lowest point in the state of Pennsylvania is located on the Delaware River in Marcus Hook in Delaware County, where it flows out of Pennsylvania and into Delaware.  The highest point in Delaware County is 500 feet at two points southeast of Wyola in Newtown Township .
Waterways in Delaware County generally flow in a southward direction and ultimately drain into the Delaware River.  The waterways are, from west to east: the Brandywine River (forming a portion of the county's western boundary with Chester County), Naaman's Creek, Stoney Creek, Chester Creek, Ridley Creek, Crum Creek, Muckinipates Creek, Darby Creek and Cobbs Creek (forming a portion of the county's eastern boundary with Philadelphia).  Crum Creek was dammed in 1931 near Pennsylvania Route 252 to fill Springton Lake (also known as Geist Reservoir), an approximately 391-acre (1.58 km²) drinking water reservoir maintained by Aqua America, the county's largest lake.
The Trainer Refinery and the Port of Chester are located along the shores of the Delaware River.
Delaware County, Pennsylvania