The aim of this study was to assess the prevalence of pet ownership among adolescents in Great Britain and identify any sociodemographic differences between pet owners and non-pet owners. A total of 14,328 11-to 15-year-old adolescents from England, Scotland, and Wales were included in the analysis. Results revealed 15-year-old adolescents were significantly more likely than 11-year-old adolescents to own dogs (OR = 1.146, p < 0.001) but less likely to own fish, reptiles, or amphibians (OR = 0.629, p < 0.001), and small mammals (OR = 0.630, p < 0.001). Thirteen-year-olds were significantly more likely than 11-year-olds to own dogs (OR = 1.240, p = 0.021) and birds (OR = 1.299, p = 0.010), but significantly less likely to own fish, reptiles, or amphibians (OR = 0.795, p < 0.001). No gender differences were found. White adolescents were more likely than non-white adolescents to own all pet types. Those living in single-parent families were significantly more likely than those living with two parents to own dogs (OR = 1.186, p = 0.013) and cats (OR = 1.319, p < 0.001). Furthermore, those who reported living in stepfamilies were also more likely to own cats (OR = 1.428, p < 0.001). Adolescents with siblings were more likely to own cats (OR = 1.391, p = 0.001), fish, reptiles, or amphibians (OR = 1.220, p = 0.037) than adolescents without siblings. Adolescents with employed parents (both or one) were significantly more likely than those with unemployed parents to own dogs (OR = 1.414, p = 0.002) and birds (OR = 1.523, p = 0.018). Adolescents from high-affluence families were less likely than adolescents from low-affluence families to own dogs (OR = 0.888, p = 0.037), small mammals (OR = 0.832, p = 0.005), and birds (OR = 0.801, p = 0.046). Furthermore, family affluence differences were found in different pet types. Differences in all pet types and siblings were also found in a proxy measure of attachment to pets. This study provides evidence that pet ownership is related to several sociodemographic factors. These are relevant to take into account when performing HAI studies on adolescents.