<p>The growing challenge of plastic waste pollution and the depletion of natural mineral resources in the construction sector represent critical environmental issues. This innovative research specifically addresses this dual problem by proposing a high-value recycling pathway for polyvinyl chloride (PVC) waste into plaster-based mortar composites, evaluating their potential for sustainable building applications. This study explores the use of recycled PVC as a substitute for natural sand in plaster-based mortar composites (PMPs), with replacement ratios ranging from 0 to 140% by volume of sand. The control plaster mortar used a plaster: sand: water ratio of 1:0.5:0.6 (by weight). The mechanical (compressive and flexural strength), physical (density, surface hardness—Shore C, capillary absorption, wetting–drying cycles, and ultrasonic pulse velocity), thermal (thermal conductivity, thermal diffusivity, and volumetric heat capacity), and acoustic properties of the developed mortars were systematically evaluated. Microscopic analysis and adhesion tests on concrete and brick substrates were also conducted to assess performance under realistic conditions. Results indicate that increasing PVC content reduces the density and ultrasonic velocity of the mortars while significantly enhancing thermal and acoustic insulation. A notable reduction in capillary absorption is attributed to the low porosity and hydrophobic nature of PVC, improving overall durability. Despite a moderate decline in mechanical strength, all formulations retained acceptable structural behavior for non-load-bearing applications. However, wetting–drying cycles lead to moderate and acceptable reductions in mechanical strength and mass. Adhesion performance decreased by 49% on concrete blocks and 41% on bricks at the maximum PVC content of 140% but remained within international standards. The mix with the highest PVC content demonstrated the most favorable balance, with thermal conductivity and sound transmission reduced by 85% and 52%, respectively, compared to the control mortar. These properties, along with the lightweight nature of the material, support its use in non-structural applications such as interior walls and ceilings. The recycling and recovery of plastic waste in this manner contribute significantly to reducing environmental accumulation, limiting the carbon footprint, and promoting a circular economy.</p>