There is a long history of conflating sightings of unidentified marine objects (UMOs) with purported sea serpents. Most sightings are either of an extremely brief duration or made by a single observer, and thus are often easy to dismiss. This is not the case, however, with respect to the so-called Gloucester Sea Serpent which frequented the Massachusetts and New York coasts during the early nineteenth century. Witnessed by hundreds, and possibly thousands, of people for extended periods repeatedly over many days, the Gloucester UMO is the most sighted ‘sea serpent’ in history. As well, due to being the object of study at the time and shortly thereafter by naturalists, the mysterious creature remains the most thoroughly investigated of all putative sea serpents. For these reasons, it has achieved an exalted status among cryptozoologists who maintain it represents the best evidence for the existence of sea serpents. For the first time, an eminently qualified aquatic biologist and ethnozoologist presents the definitive history of the phenomena, and carefully examines the evidence. It is concluded that the most parsimonious explanation behind the Gloucester Sea Serpent is as early evidence for what is today recognized as being one of the most serious threats to marine biodiversity: entanglement in fishing gear and other maritime debris. Therefore, although entanglement is widely considered to be restricted to the advent and widespread use of non-degradable plastic in the middle of the twentieth century, this new interpretation of the Gloucester UMO suggests that this particular repercussion of marine pollution has a much longer environmental history than has been commonly believed.