Biodegradable metal matrix composites (MMCs) with pure magnesium as the matrix and bioceramic calcium silicate (CS) as the reinforcement phase were fabricated by means of spark plasma sintering (SPS). The microstructure, mechanical properties and degradation behavior of the composites as well as the cellular responses to these composites were investigated. The formation of CS networks in the Mg matrix was observed when the CS content reached 20%. Among the composites with CS weight percentages ranging from 10% to 40%, the composite containing 20% CS possessed the highest structural compactness and compressive strength. Immersion tests in simulated body fluid (SBF) revealed that the Mg-20% CS composite exhibited a substantially enhanced corrosion resistance as compared with pure Mg, which was attributed to the formation of a hydroxyapatite (HA) layer on the surface as a result of the presence of the CS networks throughout the Mg matrix. In addition, ionic products from the interaction between the Mg-20% CS composite and SBF brought about a significant stimulatory effect on the alkaline phosphate (ALP) expression of MC3T3-E1 osteoblast cells. Our results indicate that CS is an effective reinforcement phase to improve both the corrosion resistance and bioactivity of Mg and the Mg-CS composites developed in this research are able to overcome the inherent drawbacks of magnesium as a biodegradable implant material.