Ferroelastic (90 degrees) domain wall motion occurs readily in bulk samples of displacive ferroelectrics such as Pb(Zr, Ti)O-3 (PZT), dictating critical piezoelectric, dielectric, and polarization switching properties. Many prior studies have used converse piezoelectric measurements to probe the dynamics of ferroelastic domains in thin films; however, such experiments are strongly influenced by the mechanical clamping effect of the substrate, which inhibits electric field-induced 90 degrees domain wall motion. Nevertheless, these observations raise a tantalizing question: Does the application of mechanical stress, rather than electric field, result in an entirely different response in thin films? Here we report biaxial stress-driven crystallographic reorientation of (100)/(001) textured, 70 nm thick Pb(Zr0.25Ti0.75)O-3 films via 90 degrees domain wall motion, measured in situ by both x-ray diffraction and piezoforce microscopy. Visual evidence of nanoscale mechanisms that underlie the direct piezoelectric effect is shown. Mobile 90 degrees domain walls effect complete orientation switching in the grains in which they operate, without apparent wall pinning, indicating that bulk-like ferroelastic behavior can extend to nanocrystalline films in the absence of substrate clamping.