Narrow-band gap, layered chalcogenide semiconductors provide a rich area of research for designing highly efficient thermoelectric materials for near-room-temperature applications due to their intrinsically low lattice thermal conductivity and multivalley electronic band structure. Here, we report on a comprehensive theoretical and experimental investigation of the thermoelectric properties of one member of this class, SnBi₂Te₄, in the temperature range of 5–700 K. Polycrystalline samples crystallize with a rhombohedral crystal structure described in the R3̅m space group (No. 166) with room-temperature lattice parameters a = 4.398(1) and c = 41.615(1) Å. To probe the electronic tunability of SnBi₂Te₄, the synthesis of a Sn-deficient sample Sn_0.95Bi₂Te₄ was attempted but led to an intergrowth of SnBi₂Te₄ and SnBi₄Te₇, indicating that only a very limited range of Sn-deficiency can exist. Transport property measurements combined with electronic band structure calculations show that SnBi₂Te₄ is a p-type, narrow-band gap semiconductor with a multivalley valence band structure and a strong asymmetry between the valence and conduction bands. The presence of Sn_Bi antisite defects, supported by Mössbauer spectroscopy, plays an important role in shaping the electronic transport properties through a decrease in the electronic band gap and a reduction in the hole mobility, which causes the onset of bipolar conduction near 350 K. Some degree of anisotropy in the electrical resistivity and thermal conductivity measured parallel and perpendicular to the pressing direction exists, while the thermopower remains nearly isotropic. The complex crystal structure of SnBi₂Te₄ contributes to the very low lattice thermal conductivity of the order of 0.5 W m^–1 K^–1 at 300 K, leading to peak ZT values of 0.32 at 350 and 400 K for Sn_0.95Bi₂Te₄ and SnBi₂Te₄, respectively.