From dragons and serpents to many-armed beasts that preyed on ships and sailors alike, sea monsters have terrified mariners across all ages and cultures and have become the subject of many tall tales from the sea. Accounts of these creatures have also inspired cartographers and mapmakers, many of whom began decorating their maps with them to indicate unexplored areas or areas about which little was known. Whether swimming vigorously, gamboling amid the waves, attacking ships, or simply displaying themselves for our appreciation, the sea monsters that appear on medieval and Renaissance maps are fascinating and visually engaging. Yet despite their appeal, these monsters have never received the scholarly attention that they deserve.