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In the literature, numerical heat transfer analysis of supercritical carbon dioxide (sCO 2) is often conducted in idealized tubular geometries to better understand its fundamental properties. Its sensitivity to temperature and pressure variations at supercritical conditions indicates that, unlike traditional subcritical turbulent flows in tubes, the thermal boundary condition (TBC) will have a noticeable influence on heat transfer trends. Despite this, it has become standard practice to validate sCO 2 numerical models without considering the TBCs. In this work, local heat transfer and flow characteristics of sCO 2 in cooled horizontal tubes under all three TBCs were investigated numerically. Five configurations were considered: a conjugate tube-in-tube heat exchanger, a single tube with constant wall heat flux, a walled tube with constant wall heat flux, a single tube with constant wall temperature, and a walled tube with constant wall temperature. Heat transfer coefficients along the tube axis were analyzed using local temperature and heat flux. Circumferential distributions of temperature and heat flux were assessed. Temperature, density, and velocity profiles within the flow were also considered. Despite being the most commonly compared in the literature, the heat exchanger and single tube with a uniform wall heat flux materialized the largest heat transfer coefficient difference (10%–12%) between configurations. Furthermore, the single tube with constant heat flux exhibits variations in circumferential wall temperature of up to 9 K when contrasted with the heat exchanger, which demonstrates nearly uniform trends in circumferential wall temperature. The results holistically indicate that the TBC significantly influences heat transfer trends in supercritical fluids and that careful consideration must be taken when validating numerical models involving supercritical fluids. • Thermal boundary condition (TBC) type strongly impacts supercritical fluid heat transfer. • TBCs used in supercritical CO 2 models and experiments rarely match during validation. • Careful consideration of TBC is needed for supercritical fluid model validation [ABSTRACT FROM AUTHOR]