Joppa is a former colonial town and current planning region of Harford County, Maryland, United States. Joppa was founded as a British settlement on the Gunpowder River in 1707 and designated as the third county seat of Baltimore County in 1712. The original boundaries of Baltimore County were defined in 1659 and contained all of modern day Baltimore County, Baltimore City, Harford and Cecil counties and parts of Howard, Carroll, Anne Arundel and Kent counties.