Mn3+ distribution at octahedral M1, M2, and M3 sites of piemontite and its effect on structural changes were investigated by X-ray Rietveld analysis of synthetic Ca2Al3-pMn3+pSi3O12(OH) piemontites. The material studied was synthesized from starting materials of Ca2Al3-qMn3+qSi3O12.5 + H2O in hydrothermal experiments with Pfluid of 200 and 350 MPa and temperature of 500 °C. Piemontites crystallized as single phases from q = 0.5, 0.75, 1.0, and 1.1 starting materials, whereas minor amounts of bixbyite and parawollastonite were associated with those synthesized from q = 1.5 and 1.75 starting materials. EPMA analyses of synthetic piemontites showed that the maximum Mn3+ content was 1.3(1) apfu. Site preference of Mn3+ at the octahedral sites is M3 > M1 ≫ M2. Mn3+ occupancies (g) at the M3 and M1 sites are correlated with the p-value in piemontite as gM3 = −0.20p2 + 1.00p and gM1 = 0.23p2 − 0.06p, respectively, where 0.0 ≤ p ≤ 1.3. With increasing p-value, the mean M3-O and M1-O distances of piemontites increase, but the M1-O1 distance and the O5-Si3-O6 angle change nonlinearly due to the Jahn-Teller effect. The nonlinear variations of the a and c parameters with increasing p are caused by changes in the M1-O1 distance and the O5-Si3-O6 angle, respectively. The M3 octahedra, which are more distorted than the M1 octahedra, become more tetragonally compressed with increasing Mn3+ at the M3 site, due to the substantial increase of the M3-O1 distance.