The Pioneer Courthouse is a federal courthouse in Portland, Oregon, United States. Built beginning in 1869, the structure is the oldest federal building in the Pacific Northwest, and the second-oldest west of the Mississippi River. Along with Pioneer Courthouse Square, it serves as the center of downtown Portland. It is also known as the Pioneer Post Office because a popular downtown Portland post office was, until 2005, located inside. The courthouse is one of four primary locations where the United States Court of Appeals for the Ninth Circuit hears oral arguments. It also houses the chambers of the Portland-based judges on the Ninth Circuit.