The intriguing crystal structure of type material trimounsite-(Y), (Y,REE)2Ti2SiO9, a = 12.293(2), b = 11.124(2), c = 4.8610(10) Å, β = 95.69(3)°, V = 661.5(2) Å3, Z = 4, was determined using single-crystal diffractometer data (MoKα X-radiation, CCD area detector) and has been refined in space group P21/c to R(F) = 3.45 % for 1821 ‘observed’ reflections with F0 > 4σ(F0). The unique structure contains two non-equivalent Ti and (Y,REE) sites (REE = rare earth element), one Si and nine O atoms. The dominant element of the structure is a spiral-like chain composed of edge-sharing, strongly distorted TiO6 octahedra. Each octahedron shares four edges with four neighbouring octahedra. The chain can also be thought of as composed of edge-sharing Ti2O10 dimers in a obliquely staggered arrangement. Ti-O distances in both TiO6 octahedra are similar and exhibit a range from 1.759 to 2.179 Å. Shortest Ti-Ti distances in the chains range between 3.098 Å (2x Til-Til), and 3.242 and 3.277 Å (Til-Ti2). The chains are aligned parallel to , i.e., the morphological elongation. They are decorated, in a staggered arrangement, with isolated, slightly distorted SiO4 tetrahedra. These are connected to two TiO6 octahedra via a common O ligand. The two non-equivalent (Y,REE) sites separate the chains from each other along both the a -and b-axis. The two sites are seven-fold coordinated, and occupied by Y atoms which are each substituted by REE (predominantly Dy, Er, Yb) to a very similar degree (∼ 24 %). A titanate chain identical to that in trimounsite-(Y) is found in Ag2TiO3, while a closely related vanadate chain is present in Ag4-xV4O12. The structure is only remotely similar to those of the known synthetic rare earth titanosilicates and chemically related minerals.