Blue Mountain, Blue Mountain Ridge, or the Blue Mountains of Pennsylvania is a ridge of the Appalachian Mountains in the U.S. state of Pennsylvania. Forming the southern and eastern edge of the Ridge-and-Valley Appalachians physiographic province in Pennsylvania, Blue Mountain extends 150 miles (240 km) from the Delaware Water Gap on the New Jersey border in the east to Big Gap in Franklin County, south-central Pennsylvania, at its southwestern end.

Some distant view of “the Blue Mountain” dominates the southern tier of most eastern and central Pennsylvanian counties, providing an ever-visible backdrop cutting across the northern or western horizon. Most transport corridors and road beds piercing the barrier necessarily pass through either large water gaps (west to east: the Susquehanna, Schuylkill, Lehigh and Delaware River valleys) or wind gaps, low gaps in the ridge caused by ancient watercourses. The barrier ridge forms a distinct boundary between a number of Pennsylvania’s geographical and cultural regions.