The hydrostatic pressure dependence of the polarization-induced interface charge in an AlGaN/GaN heterostructure is investigated via capacitance-voltage (C-V) characterization of the two-dimensional electron gas (2DEG) at the heterojunction. The linear pressure coefficient of (3.3±0.4)×1011 cm-2 GPa-1 of the polarization-induced interface charge as obtained via C-V measurements of the investigated (28 nm Al0.26Ga0.74N)/GaN heterostructure agrees well with existing theoretical and experimental data (obtained via Hall-effect measurements). It is demonstrated that C-V characterization can be applied even when Hall-effect measurements of the 2DEG are inconclusive because of parallel conduction via the underlying GaN layer. In addition, we discuss the influence on the 2DEG concentration of interface trap states, which introduce a dependence of the measured 2DEG concentration on C-V test frequency. It is shown, however, that this effect does not influence the value of the pressure coefficient of the polarization-induced interface charge as determined via C-V characterization. Collaterally, the applicability of capacitance measurements of AlGaN/GaN heterostructures for pressure sensing applications is confirmed.