Lopi (Icelandic: [ˈlɔːpɪ]) is knitting wool made from the fleece of Icelandic sheep. The fleece is made up of two layers, each with a different kind of wool. The wet-resistant outer coat contains long, coarse fibres, while the insulating layer beneath consists of soft, short fibres. These are processed together to create lopi roving and yarn.

The machine-carded roving is produced in disc-shaped rolls. This is the original unspun lopi first used for knitting c.1920s. More recently, lightly spun lopi yarn in different thicknesses has become available.

Most wool produced in Iceland is processed by Istex, the Icelandic Textile Company. They manufacture 3 types of spun lopi yarn and also unspun lopi. It comes in a variety of natural fleece shades, and in a range of dyed colours. 

Characteristic Icelandic lopapeysa sweaters are generally made from the thicker lopi yarns.