Tai To Yan, Mountain summit in northern New Territories, Hong Kong.

Tai To Yan is a 566-metre mountain in Lam Tsuen Country Park, with a main peak and the 480-metre Pak Tai To Yan to the north. Its ridge is flanked by cliffs and steep slopes, and the main summit carries a Geodetic Survey Section II triangulation station numbered 415.

Tai To Yan was originally called Tai To Yan (剃刀屻, 'Razor Ridge'), with local names Wang Toi Shan and Tsim Shan Tung. British colonial officials recorded the Hakka pronunciation of 剃 as 'Tai', and later the English map name 'Tai To Yan' was back-translated into Chinese as 大刀屻, which became the official name. The ridge was once considered one of Hong Kong's most dangerous hikes, with a narrow, sandy path and fatal falls, until metal chain railings were added at exposed sections.

The name Tai To Yan comes from a colonial-era misreading: officials recorded the Hakka name of "Razor Ridge" as "Tai", and later map readers back-translated "Tai To Yan" into Chinese as 大刀屻, "Big Knife Ridge". The old name 剃刀屻 faded, and the ridge was also miscalled 大頭羊, "Big Head Sheep".

The usual route starts at Kadoorie Farm, reached by bus 64K from Tai Po Market or Tai Wo MTR stations, and finishes at Fanling MTR station or Wah Ming Estate bus terminus. On the 64K towards Yuen Long, tapping the Octopus card again when alighting at Kadoorie Farm gives a two-dollar-sixty rebate. The ridge path is narrow with steep drops on both sides; chains and a repaired surface now cover the worst sections, but unmarked slope paths remain hazardous.

The descent from the main peak follows one of the longest continuous staircase sections found on any hiking trail in Hong Kong, dropping steeply without much break. Walkers who do the route from start to finish often find that their legs feel the stairs more than the climb by the time they reach Fanling.