A new family of environmentally friendly and low-cost membranes based on readily available mineral and polymeric materials has been developed from cast suspensions of kaolin and chitosan using aqueous phase separation and polyethylene glycol as a pore-forming agent. The as-fabricated membranes were further cross-linked with sodium tripolyphosphate (STPP) in order to strengthen the properties of the obtained samples. The functional groups determined by FTIR and EDX confirmed that the reaction occurred. A detailed study of the effects of cross-linking time on the physicochemical, surface and permeation properties showed that a 30-minute reaction enabled the composite membrane to be stable in acidic media (up to pH 2) and increased the mechanical strength twofold compared to the non-cross-linked membrane. A similar morphology to that generally observed in polymeric membranes was obtained, with a sponge-like surface overlaying a finger-like through structure. The top layer and cross-section thicknesses of the membranes increased during STPP post-treatment, while the pore size decreased from 160 to 15 nm. At the same time, the molecular weight cut-off and permeance decreased due to the increase in cross-linking density. These results observed in a series of kaolin/chitosan composite membranes showed that STPP reaction can provide control over the separation capability range, from microfiltration to ultrafiltration.