a-plane () GaN thin films grown under various conditions on r-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE) were characterized by nanoindentation and transmission electron microscopy (TEM) techniques. It was found that the increase of the built-in threading dislocation (TD) density induces an increase on the values of both the nanohardness (H) and the reduced elastic modulus (E*). Characteristic single ‘pop-in’ discontinuities were detected on the loading segments of the load–unload cycle. The TDs appear to cause a decrease of the load for the onset of the pop-in discontinuity. The dependence of the elastoplastic behaviour on the TD density is attributed to their sessile character, causing them to become obstacles for basal plane slip.