A sheet post-processing apparatus includes a first sheet conveyance passage, a second sheet conveyance passage, a sheet folding portion, and a merging portion. The second sheet conveyance passage branches off from a branch portion on the first sheet conveyance passage. The merging portion is located downstream of the branch portion on the first sheet conveyance passage, and there a folding conveyance passage of the sheet folding portion merges with the first sheet conveyance passage. When performing a folding process on a sheet, a controller guides the sheet conveyed from a sheet introduction port into the first sheet conveyance passage via the branch portion to the second sheet conveyance passage, then reverses the conveyance direction to guide the sheet to the sheet folding portion to perform the folding process, and passes the sheet through the folding conveyance passage, merges the sheet into the first sheet conveyance passage at the merging portion.