Halide perovskites are emerging as a new class of materials for thermoelectric applications owing to their low thermal conductivity and high Seebeck coefficient (thermopower). In this work, the thermoelectric parameters of vapor-deposited hybrid perovskite thin films are explored for the first time. We establish a relationship between the chemical composition and thermoelectric properties of sequentially vapor-deposited CH3NH3PbI3 films. A composition-dependent grain size and in-plane electrical conductivity evolution is observed and its influence on thermoelectric properties is analyzed. An ultralow in-plane thermal conductivity of 0.32 ± 0.03 W m–1 K–1 at room temperature is recorded for CH3NH3PbI3 using a chip-based 3ω method. Thermal conductivity measurement of a series of CH3NH3PbI3 films reveals that the thermal transport is governed by the Pb–I lattice at room temperature. Furthermore, n- and p-type CH3NH3PbI3 films achieved by compositional tuning exhibit high negative (6500 μV/K) and positive (5500 μV/K) thermopower.