Common Loons (Gavia immer) in the northeastern United States are exposed to a wide variety of environmental and anthropogenic stressors. The use of non-lethal biomarkers to assess physiological condition and population health of loons can aid in the management of this species of concern, however, many of the biomarkers currently used in field studies are influenced by capture and handling stress. Heat shock protein 70 (HSP70) is found within the red blood cells of most vertebrates and has a well-known role in mediating the cellular stress response. The objective of this study was to provide the first evaluation of HSP70 as a potential biomarker of physiological condition in free-living Common Loons. Our results indicate that: (1) similar to other avian species, HSP70 is detectable in the blood cells of Common Loons; (2) HSP70 abundance does not vary significantly with age, sex, or blood mercury concentrations, and (3) elevated abundance of HSP70 may be associated with anemia. Our results also confirm that adults and hatch-year loons exhibit differences in several blood parameters including hemoglobin, hematocrit, plasma glucose, and mercury concentrations. Together, we suggest that HSP70 abundance in blood cells may be a useful biomarker of physiological condition in free-living Common Loons, especially when used in combination with other well-established blood biomarkers.