In the biblical Noah's Ark story, the ark's door was in its side.
Genesis gives Noah instructions to make a door in the side of the vessel. The passage also describes the ark's roof and three decks, so the door is one element of a broader construction description rather than an incidental detail.
The text does not say whether the door was centered, how large it was, or what mechanism was used to open and close it. It later says that Noah and those with him entered through the door, and that the Lord shut him in. Artistic depictions consequently vary in the placement and appearance of the entrance.
Readers sometimes assume a door at the front because many modern boats and ships are pictured that way. Genesis specifically says “in its side,” however, and that wording is the basis for the canonical answer. The passage also does not identify a separate stern or bow.