The crystal structure of monoclinic KYb(WO4)2(KYbW) crystals has been refined (in space groupC2/c) at room temperature by using single-crystal X-ray diffraction data. KYbW undoped crystals were grown by the TSSG (top-seeded-solution growth) slow-cooling method. The crystals show {110}, {\bar{1}11}, {010} and {310} faces, which basically define their habit. The linear thermal expansion tensor has been determined and the principal axis with maximum thermal expansion (\alpha ^{\,\prime}_{33} = 16.68 × 10−6 K−1), {\bf X}^{\,\prime}_{3}, was located 12° from thecaxis. Its principal {\bf X}^{\,\prime}_{1}, {\bf X}^{\,\prime}_{2} and {\bf X}^{\,\prime}_{3} axes are [302], [010] and [106] directions, respectively, in the crystallographic system. The optical tensor has been studied at λ = 632.8 nm at room temperature; two principal axes,NgandNm, are located in theacplane, while the other,Np, is parallel to [010]. The principal axis with maximum refractive index (ng= 2.45),Ng, was located 19° from thecaxis.