Piezoresponse force microscopy was conducted on a polished slice of the perovskite oxynitride ceramic BaTaO2N after ammonolysis at a bias voltage of ±10 V. The ceramic surface was relatively porous because it was readily hydrolyzed by ambient humidity. While there was no piezoresponse on the surface of the ultrathin slice, a piezoresponse was clearly observed just below the surface of a thick slice. The response was enhanced in a denser sample. It increased with measuring voltage up to 7 V, and then gradually decreased up to 9 V. The response also gradually decreased with time but persisted even after 150 min at a measuring voltage of 6 V. Microstructural characterization revealed well-sintered grains, about 100 nm in size, in a relatively dense BaTaO2N structure. High-porosity BaTaO2N did not show any piezoresponse because its porous surface was easily hydrolyzed. A weak piezoresponse was observed for an electrically conductive slice obtained from deep inside a relatively dense BaTaO2N ceramic after ammonolysis.