Inspired by the prestressing principle of tempered glass, this study proposes a novel strengthening method for mortar that establishes internal compressive stress without the use of steel reinforcement. This is achieved by restraining the expansion of an expansive surface layer through a shrinking substrate. The resulting pre-compressive stress significantly enhances the flexural performance of the mortar. Experimental results demonstrate that the flexural strength of unreinforced prestressed mortar (UPM) increases with the cross-sectional height ratio. When 6% expansive agent is incorporated into the surface layer and the height ratio reaches 4, the flexural strength improves to 11.24 MPa. As the expansion rate of the surface layer increases, the flexural strength of UPM continues to rise. For instance, with a 10% expansive agent, the 28 d flexural strength increases by up to 45%. Furthermore, by employing the casting method with time interval combined with surface roughening and the use of an interfacial adhesive, the flexural strength of UPM can reach 12.33 MPa. The maximum flexural-to-compressive strength ratio approaches 1∶3. These findings highlight the critical role of interfacial bonding quality in enhancing flexural performance—stronger bonding leads to greater strength gains. The existence of internal prestress is further verified through stress release monitoring during specimen cutting, where a maximum released stress of 1.68 MPa is recorded. However, it is important to note that the cutting method only releases a portion of the internal stress. As some prestress remains unreleased, the actual prestress present in UPM is likely higher than the measured value. Therefore, the released stress should not be regarded as the absolute value of the applied prestress. Although a correlation exists between stress relief and flexural strength improvement, the two are not equivalent. In practical applications, UPM should be evaluated and selected based on the overall enhancement in flexural strength rather than the measured released stress alone. Mechanistically, the expansion of the surface layer induces tensile stress in the substrate and compressive stress in the surface layer. When the surface layer is relatively thick, the compressive prestress decreases with depth along the normal direction of the interface. This stress gradient resembles that observed in tempered glass, where compressive stress is lower at the edges than at the center, rendering edge regions structurally more vulnerable. To better understand and optimize the internal stress distribution, stress redistribution, and failure mechanisms under external loading, finite element simulations are recommended. These analyses can offer theoretical insights that support the practical design and application of UPM in engineering practice. Overall, this study introduces a new approach for mortar toughening and strength enhancement, providing a foundation for the development and application of unreinforced prestressing technology in thin-walled structures and structural repair projects.