A key challenge for lithium metal batteries is the development of a solid electrolyte with a combination of high ionic conductivity and high modulus. The development of composite electrolytes, where multiple materials provide multiple functionalities, provides a promising approach to meet all needs. Here, we developed NASICON-based composite electrolytes though a cold sintering process that utilizes a small amount of solvent and uniaxial pressure to sinter ceramics at low temperatures (<150 oC). By tuning the composition of LATP-based electrolytes, conductivities at room temperature in excess of 10-4 S/cm were achieved using cold sintering; these conductivities are comparable to what is obtained from samples sintered through conventional means (T > 800 oC). SEM images of fracture surfaces of composite electrolytes suggest strong cohesion between grains that is almost identical with that of neat LATP. Cycling of lithium half cells shows no evidence of dendrite growth, where current densities of 0.1 mA/cm2 show good cycling stability over 700 hr.