In the Genesis flood story, Noah built an altar after leaving the Ark.
Genesis 8:20 says that Noah built an altar to the Lord after the ground had dried and he and his family left the Ark. He then took some clean animals and clean birds and offered burnt offerings on the altar. The passage presents this act of worship as Noah’s first recorded action outside the vessel.
The altar scene marks the transition from survival during the Flood to a renewed relationship between God and humanity. After the offering, Genesis says God promised not to destroy every living creature in the same way again and established the covenant signified by the rainbow in the following chapter.
People sometimes remember the rainbow as the first post-Flood event, but Genesis places Noah’s altar before the covenant sign. Other traditions and modern retellings add houses, settlements, or ceremonies that are not the first action specified in Genesis. The canonical answer is therefore “an altar.”