Viili (Finnish) is a mesophilic fermented milk product found in the Nordic countries, particularly Finland.

Viili is similar to yoghurt or kefir, but when left unmixed, its texture is malleable, or "long".

This cultured milk snack resembles yoghurt and is the result of microbial action of lactic acid bacteria (LAB) and a surface-growing yeast-like fungus Geotrichum candidum present in milk, which forms a velvet-like surface on viili.

In addition, most traditional viili cultures also contain yeast strains such as Kluveromyces marxianus and Pichia fermentans.

In modern practice, pasteurized milk is used, fermentation is carried out in a dairy plant in controlled conditions using laboratory-grown cultures and the product sold fresh.