In Genesis, Noah built an altar after leaving the Ark. Genesis 8:20 says that Noah built the altar and offered burnt offerings from animals and birds described as clean. This occurs after the waters recede and Noah, his family, and the animals leave the vessel.
The altar episode marks the transition from survival during the Flood to life after the Flood. It is followed by God's covenant with Noah, the promise concerning future destruction by flood, and the rainbow as its sign. The narrative therefore places Noah's altar at a major turning point.
This detail is sometimes confused with the Ark's construction, which Genesis attributes to Noah under divine instruction. Noah is associated with both acts, but they happen at different points: he builds the Ark before the Flood and an altar after leaving it. Genesis does not name a different family member as the altar builder.