Dinas Cromlech or Dinas y Gromlech is a distinctive rhyolite rock outcrop at the Llanberis Pass, in Snowdonia, northwest Wales, which has a distinctive "open book" shape that is clearly visible from the road (A4086), and is very popular location for rock climbers and contains some of Britain's most famous and notable rock climbing routes, several of which are important in the history of rock climbing.

The obvious traditional climbing route up the deep ninety-degree angled corner resisted attempts for many years until it was climbed by Joe Brown in 1952, and called Cenotaph Corner (1952, E1 5c, with Doug Belshaw), and is regarded as one of Britain's most famous rock climbing routes.

The outcrop is an important rock climbing venue in Britain, and the corner includes some of the famous traditional climbing routes in British rock climbing history, including Cemetery Gates (E1 5b) by Don Whillans in 1951, Left Wall (E2 5c) by Ron Moseley in 1956, Right Wall (E5 6c) by Pete Livesey in 1974, and Lord of the Flies (E6 6a) by Ron Fawcett in 1979. It also includes some of the most intimidating traditional climbs in Britain, such as Steve Mayer's 1992 climb, Nightmayer (E8 6c), from which climbers risk very large falls.

In fiction, the travel writer Eric Newby gives a comic description of his first climb - and almost his only training in mountaineering - at the outcrop, the 'Spiral Stairs’. This is often referred to as Ivy Sepulchre but careful reading of Newby’s vague text will clearly show that this was indeed Spiral Stairs.He was led we are told by two expert waitresses from the inn where he is staying, in his book A Short Walk in the Hindu Kush.

• Fawcett FFA of Lord of the Flies (E6 6a): Sid Perou (director) (1980). First Ascent (Episode 3) (Motion picture). BBC. Retrieved 5 January 2022.

• North Wales Classics (Jack Geldard), 2010, Rockfax. ISBN 978-1-873341-17-9.
• North Wales Climbs (Mark Reeves, Jack Geldard, Mark Glaister), 2013, Rockfax. ISBN 978-1-873341-82-7.

• Clogwyn Du'r Arddu, a mountain crag in Snowdonia
• Dinorwic quarry, a slate climbing area near Llanberis
• History of rock climbing

• Dinas Cromlech, UKClimbing Crag Guide (2022)
• Dinas Cromlech, PlanetMountain Crag Guide (2022)