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Simple Summary: Delphinids are top predators and display long lifespans, making them susceptible to accumulating harmful environmental contaminants within their tissues, which may lead to potential adverse effects on their physiology and overall health. This study aimed to investigate exposure to phthalates in aquarium-based bottlenose dolphins and killer whales and explore potential correlations with individual physiological responses while accounting for sex, age, and reproductive stage. We found that all individuals were exposed to phthalates, including newborn dolphins. Phthalates were also correlated with aldosterone concentrations in killer whales, suggesting that this class of contaminants may exert disruptive effects on the endocrine system and metabolism of dolphins. This study could help us better understand the physiological mechanisms and effects of phthalate exposure in delphinids, which could lead to stricter contaminant discharge regulations, improving both human and aquatic/marine health. There is growing concern about the potential adverse health effects of phthalates (PAEs) on human health and the environment due to their extensive use as plasticizers and additives in commercial and consumer products. In this study, we assessed PAE concentrations in serum samples from aquarium-based delphinids (Tursiops truncatus, n = 36; Orcinus orca, n = 42) from California, Florida, and Texas, USA. To better understand the physiological effects of phthalates on delphinids, we also explored potential correlations between phthalates and the biomarkers aldosterone, cortisol, corticosterone, hydrogen peroxide, and malondialdehyde while accounting for sex, age, and reproductive stage. All PAEs were detected in at least one of the individuals. ΣPAE ranges were 5.995–2743 ng·mL−1 in bottlenose dolphins and 5.372–88,675 ng·mL−1 in killer whales. Both species displayed higher mean concentrations of DEP and DEHP. PAEs were detected in newborn delphinids, indicating transference via placenta and/or lactation. Linear mixed model results indicated significant correlations between aldosterone, month, location, status, and ΣPAEs in killer whales, suggesting that aldosterone concentrations are likely affected by the cumulative effects of these variables. This study expands on the knowledge of delphinid physiological responses to PAEs and may influence management and conservation decisions on contamination discharge regulations near these species. [ABSTRACT FROM AUTHOR]