Botanic gardens host diverse living plant collections and are increasingly recognized as sentinel sites for documenting insect biodiversity and detecting biological invasions. Aphids (Hemiptera: Aphididae) are well suited to such monitoring due to their close host associations, rapid population growth, and importance as horticultural pests and virus vectors. Here, we document the aphid fauna recorded in five European botanic gardens—Zabrze (Poland), Meise (Belgium), and the Royal Botanic Gardens in Edinburgh, Benmore, and Logan (UK)—based on surveys conducted in 2022 and 2023. Sampling approaches included short-duration expert bioblitz-style surveys and extended seasonal monitoring. In total, more than one hundred aphid species were recorded across all sites. Observed species lists differed among gardens and survey periods, reflecting variation in sampling timing, intensity, and host plant composition. Several alien aphid species to Europe of Oriental and Nearctic origin were detected, including multiple new national records. Alien taxa were found both in outdoor living collections and in controlled environments such as glasshouses, nurseries, restricted areas, and plant shops, which may function as entry points as well as locations for early intervention. These findings illustrate the value of botanic gardens for documenting aphid diversity and supporting early detection of non-native species relevant to plant health and biosecurity.