The purpose of this study was to develop and pilot-test an innovative behavioral intervention in adolescents with type 1 diabetes mellitus (T1DM) incorporating structured care of a pet to improve glycemic control. Twenty-eight adolescents with A1C > 8.5% (69 mmol/mol) were randomly assigned to either the intervention group (care of a Betta splendens pet fish) or the control group (usual care). Adolescents in the intervention group were given instructions to associate daily and weekly fish care duties with diabetes self-management tasks, including blood glucose testing and parent-adolescent communication. After 3 months, the participants in the intervention group exhibited a statistically significant decrease in A1C level (-0.5%) compared with their peers in the control group, who had an increase in A1C level (0.8%) (P = .04). The younger adolescents (10-13 years of age) demonstrated a greater response to the intervention, which was statistically significant (-1.5% vs 0.6%, P = .04), compared with the older adolescents (14-17 years of age). Structured care of a pet fish can improve glycemic control in adolescents with T1DM, likely by providing cues to perform diabetes self-management behaviors.