According to Genesis, a door was built into the side of Noah’s Ark.
Genesis 6:16 tells Noah to make an opening for light and to place the Ark’s door in its side. The passage describes the door as part of the vessel’s original construction, not as an entrance added after the flood began.
Genesis 7:16 later says that Noah, his family, and the animals entered the Ark, and that the Lord shut them in. This gives the door a narrative role as well as a practical one: it marks the transition from preparation to the flood itself.
The text does not specify the door’s dimensions, hinges, locking mechanism, or exact position beyond saying it was in the side. Illustrations often show a large central gate, but that visual convention goes beyond the information supplied by Genesis.