Young-onset dementias pose a major challenge to both clinicians and researchers. Cognitive decline may be accompanied by systemic features, leading to a diagnosis of “dementia plus” syndromes. Whipple disease is a rare systemic illness characterized by arthralgias, chronic diarrhea, weight loss, fever, and abdominal pain. Central nervous system involvement, including severe cognitive deterioration, may precede systemic manifestations, appear during the course of the disease, or even be the only symptom. We report a previously highly functional 48-year-old man whom we first suspected of having early-onset neurodegenerative dementia but then diagnosed with Whipple disease based on a detailed clinical and laboratory evaluation. Initial neuropsychological evaluation revealed marked impairment in the patient’s fluid intelligence and severe cognitive deficits in his information processing speed, complex attention, memory, visuomotor and construction dexterities, problem solving, and executive functions. At neuropsychological follow-up 21 months later, his information processing speed had improved only slightly and deficits persisted in his other cognitive functions. Repeat brain magnetic resonance imaging at that time showed that he had responded to antibiotic treatment. Because Whipple disease can cause young-onset “dementia plus” syndromes that may leave patients with neurocognitive deficits even after apparently successful treatment, we recommend comprehensive neuropsychological assessment for early detection of residual and reversible cognitive processes and evaluation of treatment response.