Genesis 8 says Noah built an altar after leaving the Ark.
The chapter describes Noah’s departure once God tells him, his family, and the animals to leave the vessel. Noah then builds an altar and offers burnt offerings from animals and birds classified as clean. This scene comes before the covenant sign of the rainbow in Genesis 9.
The detail is important because it connects the end of the flood narrative with worship and thanksgiving. It also relates back to the earlier instruction to preserve extra clean animals, rather than only one pair of each kind.
Readers sometimes place the altar scene in Genesis 9 because the covenant follows soon afterward. The chapter division, however, places Noah’s exit and altar in Genesis 8, while Genesis 9 contains the covenant, the rainbow sign, and later events in Noah’s life.