The design, synthesis, and properties of a new gadolinium-based copper-responsive magnetic resonance imaging (MRI) contrast agent is presented. The sensor (GdL1) has high selectivity for copper ions and exhibits a 43% increase in r(1) relaxivity (20 MHz) upon binding to 1 equiv of Cu2+ in aqueous buffer. Interestingly, in the presence of physiological levels of human serum albumin (HSA), the r(1) relaxivity is amplified further up to 270%. Additional spectroscopic and X-ray absorption spectroscopy (XAS) studies show that Cu2+ is coordinated by two carboxylic acid groups and the single amine group on an appended side chain of GdL1 and forms a ternary complex with HSA (GdL-Cu2+-HSA). T-1-weighted in vivo imaging demonstrates that GdL, can detect basal, endogenous labile copper(II) ions in living mice. This offers a unique opportunity to explore the role of copper ions in the development and progression of neurological diseases such as Wilson's disease.