Elasto‐optic materials have extensive applications, particularly in sensors, filters, and acousto‐optic modulators in optical–mechanical‐coupled devices. Ferroelectric phenomenological theoretical analyses have shown that materials with large ferroelastic responses also have high elasto‐optic effects, and morphotropic phase‐boundary (MPB) ferroelectrics have substantial ferroelastic coupling; thus, a large elasto‐optic effect can be obtained. In this study, guided by phenomenological theories, lanthanum‐modified lead zirconate–lead titanate (PLZT) transparent ceramics close to MPB are successfully prepared using the hot‐pressing method. The elasto‐optic properties of the PLZT transparent ceramics are investigated using the half‐wave stress and the Sénarmont compensator methods. The results indicate that the PLZT ceramics have high transmittance (higher than 70% in near‐infrared) and a large elasto‐optic effect with a stress‐optic coefficient of 10.41 × 10−12 m2 N−1 (approximately three times that of SiO2). Based on these properties, intensity‐ and interference‐type optical‐stress sensors are designed and realized; better anti‐interference ability can be achieved using the interference‐type sensor. These results indicate that PLZT transparent ceramics are promising candidates for optical–mechanical‐coupled device applications. This study also substantiates that transparent ceramics are novel elasto‐optic materials, which uncover new avenues for the development of elasto‐optic materials with excellent properties.