Sessile serrated adenoma (SSA), also known as sessile serrated polyp, accounts for about 20% of serrated polyps. The majority of SSA harbors BRAF mutations, but SSA with superimposed cytologic dysplasia (either tubular-adenoma type or "serrated-type" dysplasia) often show methylation of the MLH-1 promoter, resulting in high-levels of microsatellite instability. Such lesions are believed to be the precursor of most sporadic microsatellite unstable colon cancers. Consequently, SSA should be removed entirely, and SSA with superimposed cytologic atypia should be managed similar to conventional high-risk adenomas.

Grossly, it is most commonly found in the proximal colon (including the appendix) and is usually detected as small (<5 mm), sessile and asymptomatic polyps on screening colonoscopy. Microscopically, SSA consists of glands lined by columnar cells with abundant microvesicular mucin and bland, basally located nuclei, similar to "microvesicular" hyperplastic polyps. However, unlike hyperplastic polyps, glands in SSA demonstrate architectural distortion with basal crypt dilatation and branching, forming anchor-shaped and L-shaped crypts, as well as serration extending to the lower portion of the crypts.