There are a few kings throughout history who are known for believing in or promoting monotheism (the belief in only one god). Here are some prominent examples:
* Akhenaten (Amenhotep IV) of Egypt (c. 1353–1336 BC): Perhaps the most famous example, Akhenaten instituted the Atenist religion, which focused worship exclusively on the sun god Aten. This is often considered one of the earliest known instances of state-sponsored monotheism or henotheism (belief in one primary god while acknowledging the existence of others). His religious reforms were largely reversed after his death.
* King David of Israel (c. 1040–970 BC): is a pivotal figure in the establishment of monotheism in Israelite history. As described in the Hebrew Bible, David unified the Israelite tribes and established Jerusalem as the religious and political center. He is credited with consolidating the worship of Yahweh as the sole God of Israel and his reign is regarded as the foundation for the subsequent development of Judaism.
It is important to note that defining "believing in only one god" can be complex. Some rulers might have promoted one god above all others (henotheism or monolatry) but not necessarily denied the existence of other deities. Additionally, historical interpretations of religious beliefs can vary.