Identifying sexes in birds from visual observations could be a useful and inexpensive method. While sexual dichromatism and ornaments are readily used by observers, sexual size dimorphism can also be used to identify sexes in some bird species. This study assessed the applicability of visual observation of size differences to identify sexes in adult Black Skimmers (Rynchops niger). Black Skimmers do not have sexual dichromatism; however, Black Skimmer males are skeletally larger (6.7–31.7% depending on trait) and 33.3–37.5% heavier than females. The study focused on two subspecies: Amazonian (R. n. cinerascens) and South American (R. n. intercedens) Black Skimmers. Sex identified by visually observing size differences was consistent with the sex identified at specimen preparation from examining gonads (R_GLMM = 0.996 ± 0.004). The identification of sexes from photographs using visual observation of size had a very high within- (R_GLMM = 0.995 ± 0.001) and between- (R_GLMM = 0.984 ± 0.002) observer repeatability. Non-invasive methods for identifying sex by visual observation may allow enhanced use of data from photographic datasets, citizen science projects, and surveys using direct observation or images.