Foldable play yard

A foldable playpen includes a floor assembly 11, at least one top rail assembly 12 with at least one upper beam 122, and at least one corner post 13 movably coupled to an upper beam 122. The base 11 actuates the at least one post 13 to drive the at least one top rod 122 to pivot. This drives the top rail assembly 12 to be folded when the floor 11 is folded, and unfolded when the bottom 11 is unfolded. The corner post 13 may have first and second relatively movable columns 131, 132. Upper and lower ends of the first column 131 may respectively be pivoted to the floor and adjustably connected to the top rail structure 12. Upper and lower ends of the second column 132 may respectively be movably connected to the floor and pivoted to the top rail structure 12.