Nassarius, common name nassa mud snails (USA) or dog whelks (UK), is a genus of minute to medium-sized sea snails, marine gastropod molluscs in the family Nassariidae. They are scavengers.

The Nassariidae, Nassa mud snails (US), or dog whelks (UK) are a taxonomic family of small to medium-sized sea snails, marine gastropod mollusks in the clade Neogastropoda.