Region of New Testament Northern Palestine, mainly east of the Jordan, extending from the Sea of Galilee in the north to Philadelphia in the south. It was named after the confederation of ten, largely Greek, cities in this territory.  These cities were founded by Greek soldiers of occupation after Alexander the Great's death.  After the Roman conquest, Pompey formed the league of Decapolis. 

Cities of special interest are Philadelphia (today Amman, the capital city of Jordan) and Gerasa, the latter's inhabitants being the Gerasenes. Mark 5: 1-20.

Early in His ministry, Jesus was followed by multitudes, including people from Decapolis. Matthew 4: 23-25. Later Jesus travelled through the middle of the region. Mark 7: 31-37