According to Genesis, God shut the Ark’s door after Noah entered.
Genesis 7:16 says that Noah, his family, and the animals entered as commanded, and then “the LORD shut him in.” The wording emphasizes divine action at the transition from preparation to the Flood.
The verse does not describe a key, a mechanical lock, or a human guard. “Shut it” is therefore the concise action specified by the text, while artistic portrayals often add details that Genesis does not provide.
This moment also differs from the later description of the Ark’s physical construction. Genesis mentions a door in the side of the vessel, but the narrative separately identifies God as the one who closes it after the passengers have entered.