Supported metal nanoparticles may undergo adsorbate-induced phenomena, with evident consequences for their performance in catalytic applications. CO-induced surface phenomena involving Pd nanoparticles have been documented mostly at high temperatures. In this work, we report an unusual step-shaped CO adsorption isotherm at 30 °C on Pd/Al₂O₃, associated with an unexpected increase in the heat of adsorption and with a modification in the relative abundance of the different Pd-carbonyl species. These three independent observations can be reconciled by considering a partially reversible CO-induced roughening of the Pd(100) facets, entailing the creation of novel defective sites. The detailed knowledge of the interaction of CO with Pd nanoparticles at mild temperatures in terms of strength, coverage, and adsorption mode/sites is a key to control reactivity and has analytical relevance for the correct interpretation of CO chemisorption measurements. Our experimental strategy has the potential to become a benchmark in the investigation of the adsorbate-induced dynamic behavior of supported metal nanoparticles.