The River Flesk (Irish: An Fhleisc) is a 28-mile (45 km) river in County Kerry that drains into Lough Leane at Killarney. It is typically described in three paddling sections — upper, middle, and lower — each with distinct character and difficulty.

The Upper Flesk/Clydagh section requires a good level of water to run well and can be hard on both equipment and paddler depending on conditions. The river begins with easy grade 3 paddling, but the gradient picks up as it winds away from the road. The section features several small rapids and drops throughout. The river eventually enters a gorge, which marks the take-out for the upper section and the start of the middle Flesk. The take-out is located before a bridge, and the last rapid is visible from that bridge — paddlers are advised to inspect it before heading upstream. The exit on river right passes through a farmer's field and has historically had a bull present, making river left the preferred option. There are ropes and carabiners in place to assist with the exit on river left. The section is noted as being very overgrown. Paddlers are strongly advised to park on the valley road rather than near the farmer's entrance at the put-in, and to travel in the minimum number of vehicles given the narrow bridges used by farm machinery.

The gorge entrance rapid is grade 4, curving to the left, and a swim through it is described as potentially extremely dangerous. Paddlers not intending to continue onto the middle Flesk must exit before this rapid.

The Lower Flesk runs from approximately coordinates 51.9987, -9.3388 to 52.0479, -9.5066. It is a wide river rated grade I–II and has been used for many years to introduce paddlers to moving water. It is considered suitable only for beginners.

The River Flesk supports salmon and brown trout populations. It receives a run of spring salmon early in the season, with grilse running in late May and early June.