The Cabeço da Arruda archaeological site, located in the Muge Valley (Central Portugal), is a shell-midden burial site situated in the Mesolithic period (8,500 to 6,900 cal BP). Previous research on the valley has neglected to properly document and systematize the roughly 150 burials recovered from this site which have been curated in the Museo Geológico Museum in Lisbon since the 1930s. My research focused on systematizing the burials at this museum, identifying the individuals whose boney segments preserved in a mud-based cement (breccia), and reconstructing the burial rituals associated with body position and grave environment features using an archaeothanatological method. This allowed me to address the questions: (1) Are there identifiable burial dispositional patterns at Cabeço da Arruda?; and (2) Through comparison of the similarities and differences in dispositional patterns and mortuary practices between the three main sites in the valley (Cabeço da Arruda, Cabeço da Amoreira, and Moita do Sebastião), are there identifiable characteristics throughout the entirety of the Muge Mesolithic? I determined that the most common burial disposition at Cabeço da Arruda was dorsal decubitus with a hyper-flexed lower limb which was positioned over the thorax. The most common grave features were for the body to be interred in an oval, concave pit with lateral space and external wrappings constricting the upper limb and thorax. These patterns constitute burial rituals because their repetition, with minimal individual variation, is indicative of prescribed behaviours occurring during the internment phase. When including osteobiographical data into these interpretations, no statistical pattern was identified for age (neonates, nonadults, adults) or sex (female or male) across the burial ritual data. These interpretations were consistent with the frequencies of previous burial dispositional patterns from the valley indicating that there were consistent rituals within the valley during the Mesolithic.