A ferroelastic‐like transition at ∼140 °C in a single‐crystal LaGaO3substrate was directly observed with an optical microscope at elevated temperatures. In addition, it was found for the first time that a polished surface morphology of a pseudocubic (001) LaGaO3substrate degraded after a heat cycle up to only 200 °C. This surface degradation, i.e., surface roughening, is associated with the transition at ∼140 °C, and it may possibly be the major origin of relatively low critical current density reported for a superconducting YBa2Cu3Oxfilm grown on a LaGaO3substrate.