A gennaker is a downwind sail that can be described as a cross between a genoa and a spinnaker. It is asymmetric like a genoa, but not attached to the forestay over the full length of its luff (like a spinnaker). The gennaker has a larger camber than a genoa, making it optimal for generating lift at larger angles of attack, but the camber is signficantly less than that of a spinnaker.

The gennaker is a specialty sail primarily used on racing boats, bridging the performance gap between a genoa, which develops maximum driving force when the apparent wind angle is between 35 and 60 degrees, and a spinnaker, which has maximum power when the apparent wind is between 100 and 140 degrees. Due to its geometry, the sail is less prone to collapsing than a spinnaker and does not require the use of spinnaker pole. The lines between gennaker and asymmetric spinnaker are blurry; they are both high camber downwind sails, rigged similarly to a genoa. The difference is the amount of camber, which dictates the points of sail. A gennaker is optimal for a beam reach, while an asymmetric spinnaker is optimal for a broad reach or run.