According to Genesis, God shut Noah’s Ark after Noah and his family entered.
Genesis 7:16 says that those who entered included Noah, his sons, his wife, and the wives of his sons, and then states that the LORD shut the door behind Noah. The detail places the final sealing of the ark in God’s action rather than Noah’s.
The verse also says that animals entered in pairs according to their kinds, continuing the preparation described earlier in the chapter. The closing of the door marks the transition from preparation to the onset of the Flood.
This detail is sometimes altered in children’s books and films, where Noah is shown closing the door himself. That is a reasonable dramatic image, but it is not the action assigned to Noah in Genesis. The biblical wording specifically attributes the closing to God.