A detailed study of polycrystalline indium-based In1-x□xIn2S4 (x = 0.16, 0.22, 0.28, and 0.33) thiospinel is presented (□- vacancy). Comprehensive investigation of synthesis conditions, phase composition and thermoelectric properties was performed by means of various diffraction, microscopic and spectroscopic methods. Single-phase α- and β-In1-x□xIn2S4 were found in samples with 0.16 ≤x≤ 0.22 and x = 0.33 (In2S3), respectively. In contrast, it is shown that In0.72□0.28In2S4 contains both α- and β-polymorphic modifications. Consequently, the thermoelectric characterization of well-defined α- and β-In1-x□xIn2S4 is conducted for the first time. α-In1-x□xIn2S4 (x = 0.16 and 0.22) revealed n-type semiconducting behavior, a large Seebeck coefficient (>|200|μV K-1) and moderate charge carrier mobility on the level of ∼20 cm2 V-1 s-1 at room temperature (RT). Decreases in charge carrier concentration (increase of electrical resistivity) and thermal conductivity (even below 0.6 W m-1 K-1 at 760 K) for larger In-content are observed. Although β-In0.67□0.33In2S4 (β-In2S3) is a distinct polymorphic modification, it followed the abovementioned trend in thermal conductivity and displayed significantly higher charge carrier mobility (∼104 cm2 V-1 s-1 at RT). These findings indicate that structural disorder in the α-modification affects both electronic and thermal properties in this thiospinel. The reduction of thermal conductivity counterbalances a lowered power factor and, thus, the thermoelectric figure of merit ZTmax = 0.2 at 760 K is nearly the same for both α- and β-In1-x□xIn2S4.