Helvellyn (/hɛlˈvɛlɪn/; possible meaning: pale yellow moorland) is a mountain in the English Lake District, the highest point of the Helvellyn range, a north–south line of mountains to the north of Ambleside, between the lakes of Thirlmere and Ullswater.

Helvellyn is the third-highest point both in England and in the Lake District, and access to Helvellyn is easier than to the two higher peaks of Scafell Pike and Scafell. The scenery includes three deep glacial coves and two sharp-topped ridges on the eastern side (Striding Edge and Swirral Edge). Helvellyn was one of the earliest fells to prove popular with walkers and explorers; beginning especially in the later 18th century. Among the early visitors to Helvellyn were the poets Samuel Taylor Coleridge and William Wordsworth, both of whom lived nearby at one period. Routes up the mountain permit approach from many directions.

However, traversing the mountain is not without dangers; over the last two hundred years there have been a number of fatalities. The artist Charles Gough is more famous for his death on Striding Edge in 1805 than for what he achieved in his life. Among the human feats upon the mountain, one of the strangest was the landing and take-off of a small aeroplane on the summit in 1926.

Since 2020, the summit of Helvellyn including both Striding and Swirral Edges and the wider Glenridding Common have been managed by the John Muir Trust, a wild places conservation charity in partnership with the Lake District Park Authority.

The volcanic rocks of which the mountain is made were formed in the caldera of an ancient volcano, many of them in violently explosive eruptions, about 450 million years ago during the Ordovician period. During the last ice age these rocks were carved by glaciers to create the landforms seen today. Since the end of the last ice age, small populations of arctic-alpine plants have survived in favourable spots on rock ledges high in the eastern coves. A species of alpine butterfly rare in Britain, the mountain ringlet, lives on and around Helvellyn. Mineral veins, some with deposits of the lead ore galena, exist within Helvellyn's rocks, but attempts to find sufficient quantities of lead to be worth mining have failed.

The top of Helvellyn is a broad plateau, trending roughly from north-west to south-east for about a kilometre between Lower Man and the start of Striding Edge. Throughout this distance it remains more than 900 m (3,000 ft) high. To the west the mountain is rounded, its shape before glaciation: the ground drops gently at first but then more steeply down to Thirlmere, whereas on the eastern side deep glacial coves, each backed by high cliffs, are separated by spectacular sharp ridges or arêtes. The middle of these coves contains Red Tarn. The reason for this east-west contrast has been debated.

Like much of the main ridge of the range, Helvellyn stands on the watershed between Thirlmere and the Derwent river system to the west, and Ullswater and the Eden river system to the east. Streams on the west side drain directly into Thirlmere, apart from Helvellyn Gill which flows into a parallel valley to the east of Great How and empties into St John's Beck. A leat captured the water of Helvellyn Gill, taking it into Thirlmere reservoir.

A never-failing spring called Brownrigg Well exists 90 m (300 ft) below the summit of Helvellyn, about 500 m (550 yd) due west of the highest point, at the head of Whelpside Gill at 2,800 feet. In the nineteenth century a leat was constructed to direct the water of this spring into the gill to its north to serve the needs of the Helvellyn Mine further down. This leat has now fallen into disuse. The gill it led to is not named on any map, but some authors have referred to it as Mines Gill.