In Genesis, Noah built an altar after leaving the ark.
Genesis 8:20 says Noah built an altar to the Lord and offered burnt offerings from animals and birds considered suitable for sacrifice. This is the first action attributed to Noah after he, his family, and the animals leave the ark on dry land.
The passage connects the altar with worship and thanksgiving rather than with constructing a new settlement or vessel. Genesis then describes God’s covenant with Noah in chapter 9, including the promise signified by the rainbow.
The account has an important textual detail: Noah’s earlier instructions concerned taking animals into the ark, while Genesis 8:20 specifically identifies animals and birds suitable for sacrifice. Readers sometimes compress these scenes into one general act of thanksgiving, but the altar is the concrete object named in the text.