Observation Post at Mau Wu Shan, Military observation post in Sai Kung District, Hong Kong.

The Observation Post at Mau Wu Shan is a former military watchtower in Sai Kung, Hong Kong, made up of a cylindrical stone tower and a rectangular stone shelter. The site sits on a hillside above Tseung Kwan O, facing the sea passages that run through the area.

The post was built before 1898 under the Qing Dynasty to serve as a customs station for watching over shipping and collecting taxes on goods such as opium. It was part of a wider network of coastal stations that the Qing government used to control maritime trade in the region.

The site is known locally as "Mau Wu Fei Bau" (Mau Wu Abandoned Fortress), a name used by hikers rather than its official title. The cylindrical tower once had a roof shaped like a Chinese hat, shown in old photographs; it has since disappeared and the reason is unknown. All doors and windows are gone, leaving only their stone frames.

The ruins are enclosed by wire fencing and cannot be entered; visitors view the tower and collapsed-roof hut from outside. The site is reached on foot via trails on Mau Wu Shan above Po Lam Road South. The structures stand on a concrete surface, with loose stones piled in front of the steps.

The cylindrical tower once had a second storey and a roof shaped like a Chinese hat, as shown in old photographs. The upper floor has since collapsed, leaving only a stone staircase to the top, and the reason for the roof's disappearance is unknown.