Bi5Ti3FeO15 (BTF) has recently attracted considerable interest as a typical multiferroic oxide, wherein ferroelectric and magnetic orders coexist. The ferroelectric order of BTF implies its piezoelectricity, because a ferroelectric must be a piezoelectric. However, no extensive studies have been carried out on the piezoelectric properties of BTF. Considering its high ferroelectric-paraelectric phase transition temperature (T-c similar to 761 degrees C), it is necessary to analyze the piezoelectricity and thermal stabilities of BTF, a promising high-temperature piezoelectric material. In this study, lightly manganese-modified BTF polycrystalline oxides are fabricated by substituting manganese ions into Fe3+ sites via the conventional solid-state reaction method. X-ray diffraction and Raman spectroscopy analyses reveal that the resultant manganese-modified BTF has an Aurivillius-type structure with m = 4, and that the substitutions of Fe by Mn lead to a distortion of BO6. The temperature-dependent dielectric properties and direct-current (DC) resistivity measurements indicate that the Mn ions can significantly reduce the dielectric loss tan delta and increase the DC resistivity. The piezoelectricity of BTF is confirmed by piezoelectric constant d(33) measurements; it exhibits a piezoelectric constant d(33) of 7 pC/N. Remarkably, BTF with 4 mol% of Mn (BTF-4Mn) exhibits a large d(33) of 23 pC/N, three times that of unmodified BTF, whereas the Curie temperature T-c is almost unchanged, similar to 765 degrees C. The increased piezoelectric performance can be attributed to the crystal lattice distortion, decreased dielectric loss tan delta, and increased DC resistivity. Additionally, BTF-4Mn exhibits good thermal stabilities of the electromechanical coupling characteristics, which demonstrates that manganese-modified BTF oxides are promising materials for the use in high-temperature piezoelectric sensors.