A genoa (pronounced like the city, or as jenny) is a type of large jib-sail used on sloop-rigged craft. Its large surface area increases the speed of the craft; however in high wind conditions a smaller storm jib is usually substituted.

The term genoa is often used somewhat interchangeably with jib, but technically there is a clear delineation. A jib is only as large as the foretriangle, which is the triangular area formed by the mast, deck or bowsprit, and forestay. A genoa is larger, with the leech going past the mast and overlapping the mainsail. Genoas are categorized by the percentage of overlap. This is calculated by looking at the distance along a perpendicular line from the luff of the genoa to the clew, called the LP (for "luff perpendicular"). A 150% genoa would have an LP 50% larger than the foretriangle length. Sail racing classes often specify a limit to genoa size. On International Offshore Rule boats, different classes of genoa have 150%, 130%, and 98% overlap, and so on. Under Performance Handicap Racing Fleet rules most boats are allowed 155% genoas without a penalty.

Genoas do add more sail area to a craft, but at the expense of more difficult handling. It is harder to tack a genoa than a jib, since the overlapping area can become tangled with the mast unless carefully tended during the tack. Genoas are very popular in some racing classes, since they count only the foretriangle area when calculating foresail size; a genoa allows a significant increase in actual sail area within the calculated sail area. In boats where sail restrictions are not applicable, genoas of 200% overlap can be found, although those over 150% are not often seen, since the addtional area is shadowed by the mainsail and generates diminishing returns in terms of power per actual sail area.

The gennaker is a fairly new type of sail, and is a hybrid between a genoa and an asymmetrical spinnaker. Gennakers are even larger than genoas (200% overlaps are not uncommon). They have a much greater camber than a genoa, for generating larger amounts of lift when broad reaching and running, but the large camber results in poor performance when close hauled, making the gennaker a downwind only sail. Spinnakers, on the other hand, are strictly for running, and will collapse if used to reach.