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.