Leptosperin (originally but no longer "leptosin") is a bioactive component of Mānuka honey.

It is the gentiobiose glycoside of syringic acid methyl ester.

It is named for the genus Leptospermum, the shrubs from which bees harvest nectar to make this type of honey.

[1] This bioactive component is the source of antibacterial property of Mānuka honey[2] or other Leptospermum species derived honey.

[3] Due to leptosperin being present in Mānuka honey but not other honeys, its presence can be used as a marker to confirm the authenticity of claimed Mānuka products.