Lough Ouler, Natural lake in County Wicklow, Ireland.

Lough Ouler is a mountain lake on the eastern side of Tonelagee in County Wicklow, at 627 metres above sea level. The lake lies in a glacial hollow, with Tonelagee's summit rising to 762 metres about one kilometre to the west. Woodland covers the surrounding slopes.

Lough Ouler lies in a hollow on the eastern side of Tonelagee, about 30 km south of Dublin. The source records the lake at 627 m, with the ground rising to 762 m within a kilometre to its east. The surrounding slopes are wooded, and the local climate is coastal, averaging 6 °C, with August around 12 °C and January around 0 °C.

The lake sits on the slopes of Tonelagee, a name meaning "backside to the wind" in Irish, and the mountain is a well-known Wicklow landmark. The lake's outline is often compared to a heart when seen from the surrounding heights, and walkers commonly stop at the summit cairn above it.

Lough Ouler sits at 627 metres on the slopes of Tonelagee, whose summit rises to 762 metres about 1 km to the east. The ground around the lake is wooded hillside, and the mountain climate is oceanic: about 6 °C on average, with August the mildest month at around 12 °C and January the coldest at around 0 °C.

Lough Ouler sits at 627 metres on the slopes of Tonelagee, whose summit rises to 762 metres about a kilometre to the east. The lake lies roughly 30 kilometres south of Dublin, and the surrounding hillsides are covered with deciduous woodland. Its heart shape is best picked out from the Tonelagee ridge above the water.