In the Genesis account, the first thing Noah built after the Flood was an altar.
Genesis 8:20 says that Noah built an altar to the Lord after leaving the Ark. He then took some animals and birds described as clean and offered burnt offerings on the altar. This event occurs before the covenant sign of the rainbow in Genesis 9.
The altar is sometimes confused with the vineyard Noah planted later in the same post-Flood sequence. Genesis records both events, but the altar comes first. The text also connects the offering with the animals classified as clean, a distinction already introduced in the instructions about which creatures entered the Ark.