Fresnoite, Ba2TiOSi2O7, has been predicted structurally to be a new ferroelectric. Detection of both ac and dc dielectric hysteresis verifies the prediction. The spontaneous polarization Ps estimated from the hysteresis at 1.2 MV m−1 ac is ∼0.2 C m−2 at 295 K, comparable with the minimum Ps observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19(3) J mol−1 K−1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change ∼0.2 J mol−1 K−1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are ∼60 K wide. Observation of dielectric hysteresis at 875 K shows that neither anomaly corresponds to the Curie temperature; both are likely associated with small changes in atomic position, not with symmetry changes. Melting onset in fresnoite is 1703(1) K with undercooling as deep as 435 K.

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The atomic coordinates of K2(NbO)2Si4O12 reported in space group P4bm satisfy the structural criteria for ferroelectricity. The estimated phase‐transition temperature, Tc = 2230 (250) K, is substantially in excess of Tmelting = 1476 (5) K; the hypothetical paraelectric phase in space group P4/mbm is hence experimentally inaccessible. A strong indication that the phase transition does not occur at T < Tmelting is provided by the absence of both calorimetric and dielectric permittivity anomalies below Tmelting, the linear thermal dependence of unit‐cell parameters with expansion coefficients α11 = –0.53 (12) × 10–6 and α33 = 24.66 (11) × 10–6 K–1 between 5 and 843 K, and the generation of second harmonics between 300 and 1353 K. Demonstration of dielectric hysteresis under both direct and alternating current, with spontaneous polarization Ps≃ 0.2 C m–2, provides unambiguous verification of the ferroelectric property.