Tenascin C (TNC) is an extracellular matrix protein able to modulate the immune response. Knowledge regarding its role during sepsis and general critical illness is still limited. We here assessed the temporal dynamics of plasma TNC during sepsis and nonseptic critical illness, its capacity to predict patient outcome, and its specificity toward infection. TNC plasma concentrations were measured in 895 consecutive sepsis patients on ICU admission, day 2 and 4 thereafter, and, in a subset, before ICU discharge. To assess TNC diagnostic value, we compared patients with abdominal sepsis (N = 143) to noninfectious abdominal surgery controls (N = 98), and patients with severe community-acquired pneumonia (CAP, N = 227) to patients whose CAP diagnosis was retrospectively refuted (no-CAP controls, N = 70). Plasma TNC levels were persistently elevated in sepsis patients compared with healthy volunteers throughout the ICU stay. TNC levels varied by the site of infection and increased with the number of organs failing. Association of TNC levels with 30-day mortality could be wholly attributed to differences in disease severity. Noninfectious ICU patients also showed elevated TNC levels, albeit with different temporal dynamics. Although admission TNC was higher in CAP than in no-CAP patients, it performed poorly in distinguishing the 2 groups.TNC plasma levels are persistently elevated during sepsis and nonseptic critical illness. In sepsis patients, they are reflective of disease severity more than independent predictors of mortality. Despite higher levels in patients with infection compared with noninfectious controls, TNC does not perform sufficiently to be used as a standalone biomarker discriminating sepsis from noninfectious critical illness.