This study demonstrates a method for measuring the optical absorption cross-sections (σa) of Au-Ag nanocages and Au nanorods using photoacoustic (PA) sensing. PA signals are directly proportional to the absorption coefficient (μa) of the nanostructure. For each type of nanostructure, we first obtained μa from the PA signal by benchmarking against a linear calibration curve (PA signal vs. μa) derived from a set of methylene blue solutions with different concentrations. We then calculated σa by dividing the μa by the corresponding concentration of the Au nanostructure. Additionally, we obtained the extinction cross-section (σe, sum of absorption and scattering cross-sections) from the extinction spectrum recorded using a conventional UV-vis-NIR spectrometer. From the measurements of σa and σe, we were able to easily derive both the absorption and scattering cross-sections for each type of gold nanostructure. This method can potentially provide the optical absorption and scattering properties of gold nanostructures and other types of nanomaterials.