Croagh Patrick (Irish: Cruach Phádraig, meaning '(Saint) Patrick's stack'), nicknamed 'the Reek', is a mountain with a height of 764 m (2,507 ft) and an important site of pilgrimage in County Mayo, Ireland. The mountain has a pyramid-shaped peak and overlooks Clew Bay, rising above the village of Murrisk, several kilometres from Westport.

It has long been seen as a holy mountain. It was the focus of a prehistoric ritual landscape, and later became associated with Saint Patrick, who is said to have spent forty days fasting on the summit. There has been a church on the summit since the 5th century; the current church dates to the early 20th century. Croagh Patrick is climbed by thousands of pilgrims every year on Reek Sunday, the last Sunday in July, a custom which goes back to at least the Middle Ages.

Croagh Patrick is the fourth-highest mountain in the province of Connacht on the P600 listing after Mweelrea, Nephin and Barrclashcame. It is part of a longer east–west ridge; the lower westernmost peak is named Ben Goram.

'Croagh Patrick' comes from the Irish Cruach Phádraig meaning "(Saint) Patrick's stack". It is known locally as "the Reek", a Hiberno-English word for a "rick" or "stack". Previously it was known as Cruachán Aigle or Cruach Aigle, being mentioned by that name in medieval sources such as Cath Maige Tuired, Buile Shuibhne, The Metrical Dindshenchas, and the Annals of Ulster entry for the year 1113. Cruachán is simply a diminutive of cruach meaning "stack" or "peak". Aigle was an old name for the area. The Dindsenchas (lore of places) says that Aigle was a prince of Connacht who was slain by his uncle Cromderg in revenge for his slaying of a woman under Cromderg's care. It is also suggested that Aigle is an alternative form of aicil, "eagle".

The Marquess of Sligo, whose seat was nearby Westport House, bears the titles Baron Mount Eagle and Earl of Altamont ("high mount"), both deriving from Croagh Patrick.

Perhaps because of its prominence, its pyramidal quartzite peak, and the legends associated with it, Croagh Patrick has long been seen as a holy mountain.

Archaeologist Christiaan Corlett writes that the large number of prehistoric monuments surrounding and oriented towards Croagh Patrick "suggests that the mountain has been a local spiritual inspiration since at least the Neolithic, and during the Bronze Age became the focus of an extensive ritual landscape".

A short distance east of the mountain lies the Boheh Stone, an outcrop covered with ancient rock art. There are more than 260 carvings, making it one of the most detailed pieces of ancient rock art in Ireland, and one of only two in the province of Connacht. In 1987 it was rediscovered that, from the Boheh stone, the setting sun appears to roll down the slope of Croagh Patrick in late April and late August. It is believed the stone was chosen because of this natural phenomenon. A stone row at Killadangan is aligned with a niche in the mountain where the sun sets on the winter solstice.