Lusonectes (meaning "Portuguese swimmer") is an extinct genus of microcleidid plesiosaur from the Early Jurassic (Toarcian) São Gião Formation of Portugal.

The generic name is derived from the prefix Luso, from Latin Lusitania referring to Portugal, and nektes ("swimmer" in Greek). The specific name honors Henri Émile Sauvage, who was the first person to describe the holotype specimen.

The holotype, MG33, a partial skull and articulated mandible, was discovered possibly by geologist Paul Choffat and his team during the 19th century within rocks from the São Gião Formation near Murtede, Portugal.

Henri Émile Sauvage (1898) described MG33 as belonging to an unknown species of Plesiosaurus. Other authors, including Bardet et al. (2008) and Ruiz−Omeñaca et al. (2009) also classified MG33 within Plesiosaurus. Castanhinha and Mateus (2007) and Smith & Vincent (2010) instead classified the specimen as an indeterminate member of Plesiosauria.

The specimen was described and named by Adam S. Smith, Ricardo Araújo and Octávio Mateus in 2012 as Lusonectes sauvagei. Lusonectes was described as the first diagnostic plesiosaur species discovered in Portugal to date.

It is based on a single autapomorphy, a broad triangular parasphenoid cultriform process that is as long as the posterior interpterygoid vacuities, and also on a unique character combination.

Smith, Araújo and Mateus (2012) found Lusonectes to belong to the Plesiosauridae when placed within a cladogram created by Ketchum and Benson (2010).