Adult males of Gammarus roeselii can reach a notable length of up to 22 millimeters, making them relatively large within their species. In contrast, females of this species are generally smaller in size than the males.One distinctive feature that sets G. roeselii apart from many other common amphipods is the presence of spines on their fifth through seventh pereiopods. These spines are conspicuous and serve as a distinguishing characteristic for identifying this species.The coloration of G. roeselii individuals can be quite variable. They may exhibit shades of green, brown, gray, or yellow, and in some cases, individuals have reddish markings on certain parts of their carapaces. This variability in coloration can be influenced by environmental factors and may provide some camouflage or protection against predation within their specific habitats.