This research studies the understanding of the sea in the imagination of Muslim geographers up to the eighth century AH. To this end, it reviews marine phenomena including tides, islands, terraces, and marine mines in the Islamic geographical texts. It also explores concepts related to the sea (such as the lake) and the bay - and their differences from current definitions. The purpose of this study is to recognize issues related to the sea - such as the division and naming of seas - from the perspective of Muslim geographers. Numerous researches have been done on the seas, islands, and navigation in the Islamic Middle Ages but no proper research has been done on the Muslims' perception of the world's seas. What distinguishes this study from other research is the knowledge of geographers' perceptions and views of the sea and related issues from the perspective of geographical sources. A study of geographical texts shows that the definition of bay and sea by medieval Muslim geographers differed from current definitions of these concepts. Geographers have mentioned the number of seas in the world differently based on verses from the Qur'an and influenced by Greek science and civilization. Also in geographical texts, several names have been used for a single sea.