The new ambient-temperature hexagonal (space group P6₃ /mmc) polymorph of tripotassium ytterbium(III) disilicate (β-K₃YbSi₂O₇) has been synthesized by the high-temperature flux method and subsequently structurally characterized. In the course of the temperature-dependent single-crystal diffraction experiments, a phase transformation of β-K₃YbSi₂O₇ to a novel low-temperature orthorhombic phase (β′-K₃YbSi₂O₇, space group Cmcm) has been observed at about 210 K. β-K₃YbSi₂O₇ is isostructural with K₃ErSi₂O₇, whereas β′-K₃YbSi₂O₇ adopts a new type of structure. Both compounds can be built up from a regular alternation of layers of two types, which are parallel to the (001) plane. In the octahedral layer, YbO₆ octahedra are isolated and linked by K1O₆₊₃ polyhedra. The second, slightly thicker sorosilicate layer is formed by a combination of Si₂O₇ dimers and K2O₆₊₃ polyhedra. The boundary between the layers is a pseudo-kagome oxide sheet based on 3.6.3.6 meshes. The phase transition is due to a tilt of the two SiO₄ tetrahedra forming a single dimer which induces a decrease of the Si—O—Si angle between bridging Si—O bonds from 180° (dictated by symmetry in space group P6₃/mmc) to ≃164°. Magnetic characterization indicates that K₃YbSi₂O₇ remains paramagnetic down to 2 K, showing no apparent influence of the phase transformation on its magnetic properties. Analysis of the magnetization data revealed the positions of the three lowest crystal field levels of the Yb³⁺ cations, as well as the corresponding projections of their angular momentum on the direction of the magnetic field.