This study investigates the development of fly ash cenospheres (FACs)–based lightweight cementitious composites that can be used as both structural and nonstructural material in digital construction. FACs are hollow alumino-silicate particles obtained as a by-product of a thermal power plant's coal combustion. To fabricate 3D-printable lightweight cement composites, ordinary Portland cement, fine sand, silica fume, water, FACs, superplasticizers, and viscosity-modifying agents (VMA) were used. The effects of the FACs volume ratio and VMA ratio on the rheological and thermal properties of the developed composites were studied. First, stress growth tests were conducted to determine the yield stress and apparent viscosity of the FACs-based cement mixtures with four different FACs volume ratios and four different VMA ratios. A heat flow meter technique was employed to determine the thermal conductivity and thermal resistance of the developed composites with different FACs volume ratios. Prismatic specimens were cast and cured for 28 days for thermal tests. Before thermal measurements were conducted at room temperature, the specimens were dried in an oven until they reached a constant weight. In addition, cubic specimens were prepared to determine the compressive strength of the developed composites. The morphological and microstructural characteristics of the tested compression specimens were analyzed using a scanning electron microscope. The printability of the most promising mixture proportion was assessed using a 3D concrete printer equipped with a screw pump and two different nozzles. Results showed that increasing the VMA ratio or FACs content decreased the thermal conductivity of cementitious mixtures, while the rate of this decrease was higher for increasing FACs volume ratios. By replacing 60% of sand with FACs and using VMA at 0.3% by weight of binder in a mortar composite, a printable lightweight cementitious composite with good thermal and mechanical properties could be obtained.