Lough Nacung Upper (Irish: Loch na Cuinge Uachtarach, meaning "upper lake of the isthmus" or "upper narrowness lake") is a natural freshwater lake in Gweedore, in the north-west of County Donegal, Ireland. The lake sits at a surface elevation of 59 metres (194 feet) and covers approximately 2.08 km² (0.80 sq mi), with a maximum length of 2.72 miles (4.38 km) and a maximum width of 0.38 miles (0.61 km). Its shoreline extends roughly 7.17 miles. The name reflects its geographical position as a narrow connecting body of water, linking Lough Dunlewey to the east with Lough Nacung Lower to the west, which in turn drains into the Clady River. The catchment area is 78.62 km².

The lake is designated a Special Area of Conservation, a status that may carry implications for access and use. Paddlers should be aware that the Lakeside Centre in Dunlewy operates boat hire on the connected water system, and a fee is charged for use of the water. The Lakeside Centre is a practical reference point for anyone planning a trip in the area.

The surrounding landscape includes views toward Errigal, the prominent quartzite peak that dominates the Donegal skyline to the south-east. The principal fish species are sea trout and salmon, with runs typically beginning from mid-June onward, and brown trout present throughout. The fishing season runs from February to October, and during this period paddlers should be aware of angling activity on the water, particularly fly fishing and dapping, which are the primary methods used.

A Facebook paddling group post references Lough Dunlewey and Lough Nacung together as a paddling destination, suggesting the two connected lakes are explored as a combined route. Given the lake's elongated shape and its position as a link in a chain of connected loughs, a linear paddle from Lough Dunlewey through Lough Nacung Upper toward Lough Nacung Lower is a natural route option. The coordinates for the lake are approximately 55°01′55″N, 8°09′58″W.