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Simple Summary: Photo-identification is a non-invasive method of identifying individual animals from natural markings present on one or more parts of the body. The dorsal fin of a dolphin is the portion that contains individually distinctive marks and the most commonly visible when an animal surfaces to breathe. The camera captures an image of the dorsal fin whenever it is sighted and based on the number of times an image is captured, a statistical method called capture–recapture is used to estimate the site fidelity and the size of the population in a defined area. Here, we applied these methods to study the bottlenose dolphins (Tursiops truncatus) between 2017 and 2020 at the Tiber estuary (Mediterranean Sea, Rome, Italy), a polluted major river supplying organic material to the Capitoline (Roman) coastal area and nearby regions. We identified 347 unique individuals, with different patterns of residency (i.e., the tendency of dolphins to remain in, or return to, the study area): 42 were labeled resident, 73 part-time, and 232 transient. Estimating a total population size of 529 individuals and highlighting the presence of animals with a strong site-fidelity, this study suggests developing conservation plans for this vulnerable top-predator species not only focused on areas close to the Tiber River mouths but also extended to cover a broader scale of area. Periodic assessments of population status and trends to detect natural influences and human effects on coastal dolphin are often limited by lack of baseline information. Here, we investigated for the first time the site-fidelity patterns and estimated the population size of bottlenose dolphins (Tursiops truncatus) at the Tiber River estuary (central Mediterranean, Tyrrhenian Sea, Rome, Italy) between 2017 and 2020. We used photo-identification data and site-fidelity metrics to study the tendency of dolphins to remain in, or return to, the study area, and capture–recapture models to estimate the population abundance. In all, 347 unique individuals were identified. The hierarchical cluster analysis highlighted 3 clusters, labeled resident (individuals encountered at least five times, in three different months, over three distinct years; n = 42), part-time (individuals encountered at least on two occasions in a month, in at least two different years; n = 73), and transient (individuals encountered on more than one occasion, in more than 1 month, none of them in more than 1 year; n = 232), each characterized by site-fidelity metrics. Open POPAN modeling estimated a population size of 529 individuals (95% CI: 456–614), showing that the Capitoline (Roman) coastal area and nearby regions surrounding the Tiber River estuary represent an important, suitable habitat for bottlenose dolphins, despite their proximity to one of the major urban centers in the world (the city of Rome). Given the high number of individuals in the area and the presence of resident individuals with strong site fidelity, we suggest that conservation plans should not be focused only close to the Tiber River mouths but extended to cover a broader scale of area. [ABSTRACT FROM AUTHOR]