Lough Gowna (from Irish: Loch Gamhna meaning "calf lake") is a fresh water lake which is the uppermost lake on the River Erne. It is located on the border between County Longford and County Cavan, with the largest part of the lake being in County Longford.

Lough Gowna is a moraine-dammed lake formed at the end of the last glaciation, and owes its complex indented shape to the underlying drumlin landscape. This results in a large number of bays and inlets on the lake, often connected by narrow channels. The river Erne enters the system in a western direction into the easternmost part of the lake (locally known as Derries Lough) and exits northward through County Cavan. However, the damming caused by the moraines that gave rise to the lake result in large bodies of water to the north-west and south-west which form the bulk of the area of the lake. A number of other small rivers also flow into the lake. The county border runs through the north-western part of the lake, and then turns eastwards, with the western and southern parts of the lake being in Longford and the north-eastern parts being in Cavan.

The north-western and south-western portions of the lake are connected by a narrow channel at Dernaferst (a townland on the western (Longford) shore of the lake, but which is in County Cavan). The northern and eastern shores of the lake are surrounded by peat bog, with areas of planted woodland along the southern shores of the lake in former demesnes in the townlands of Derrycassan and Culray. The lake is considered to be an important site for wintering wildfowl.