Objectives: To evaluate the correlation between ultrasonographic and infrared pupillary assessments in critically ill patients, including neurocritically ill patients. Design: Prospective, observational study. Setting: Tertiary teaching hospital intensive care unit (ICU) in Montevideo, Uruguay. Patients: Twenty-six adults patients with age 18 or older admitted to the intensive care unit with and without neurologic pathology. A total of 212 pupillary measures were made between ultrasonographic pupillary assessment (UPA) and infrared pupillary assessment (IPA). Interventions: This was a study that utilized non-invasive (minimal risk) ultrasonographic and infrared pupillary assessment in patients admitted to the ICU. Time between UPA and IPA in a single patient was consistently less than 3 min. Measurements and main results: There was a strong positive association between UPA and IPA (right eye [OD]: r = de 0.926, p-value < 0.001; left eye [OS], r = 0.965, p-value < 0.001), also observed in the group of neurocritically ill patients (OD: r = 0.935, p-value < 0.001; OS: r = de 0.965, p-value < 0.001). Taking IPA as reference measure, the percent error for all subjects was 2.77% and 2.15% for OD and OS, respectively, and for neurocritically ill patients it was 3.21% and 2.44% for OD and OS, respectively. Conclusions: Ultrasonographic pupillary assessment is strongly correlated with infrared pupillary assessment in critically ill patients, including neurocritically ill patients. Ultrasonographic pupillary assessment is a quick, feasible, non-invasive method that allows accurate pupillary assessment, particularly neurologic function, in patients in whom a more precise measurement of the pupil is required or eye opening is not possible (e.g., periorbital edema due to traumatic brain injury).