Christiaan Huygens played a key role in the Scientific Revolution, his groundbreaking research covering the fields of astronomy, mathematics, physics, natural philosophy and horology. Despite his international fame as a scientist, hitherto we knew of no more than three original portraits after life that depict him in adulthood. This intimate portrait by his brother Constantijn, secretary to Stadtholder William III and a gifted dilletante draftsman, is an important addition to the portrait iconography of Christiaan Huygens.