There has been some debate over this but I can say confidently that the longest river in Jamaica is the Rio Minho.
This was confirmed in an article, dated August 4, 2004 in the Jamaica Gleaner by Mrs. Winsome Hudson from the National Library of Jamaica.
It is approximately ninety three kilometers (93km) or fifty eight miles (58m) long, and was originally called Rio Caobana (Mahogany River) by the Spaniards. It flows from the Dry Harbour Mountains to Carlisle Bay.
It was once thought that the Black River, 30 miles (50 km) long was the longest.