It is well known that the acoustic scattering from a swimbladder fish is dominated by the gas‐filled swimbladder. In this paper, the sound backscattered by a fish swimbladder of an arbitrary geometrical shape is studied. Particular attention is paid to the scattering at low frequencies, i.e., the acoustic wavelength is much larger than the size of swimbladder. A surface average approach is presented to study the sound scattering by the fish swimbladder. A formula is derived for the low‐frequency scattering function of an arbitrary swimbladder, incorporating the familiar resonance feature of a gas‐filled scatterer. The theory is applied to a prolate spheroid gas bubble, yielding favorable results compared to that obtained by a rigorous approach. As an application, the theory is also used to study the low‐frequency acoustic scattering of some fish, and the results are compared with some existing experimental data. It is shown that the theoretical results agree reasonably well with the data for frequencies near or below resonance frequencies, but significant discrepancy remains for higher frequencies.