Guanidinoacetic acid (GAA) is formed from arginine (Arg) and is the immediate precursor to creatine (Cr) and phosphocreatine (PCr), which are important compounds involved in muscle energy homeostasis. This study sought to determine whether GAA could spare Arg in broiler chicks fed an Arg-deficient practical diet. A basal [0.84% standardized ileal digestible (SID) Arg] was supplemented with combinations of L-Arg (0 or 0.16%) and GAA (0, 0.06, or 0.12%) to form a factorial arrangement of diets; the unsupplemented basal served as the negative control (NC). Additionally, the basal was supplemented with 0.32% Arg to generate an Arg-adequate positive control (PC). Mash diets were fed to 8 replicate pens of 5 chicks per treatment from d 8 to 22 posthatch, with measurements including growth performance, blood GAA metabolites, muscle cellular energy markers, and clinical outcomes. Supplementation of 0.16% Arg increased (P < 0.05) BW gain from d 15 to 22 posthatch, while graded addition of GAA tended to improve BW gain (P < 0.094). Supplementation of either Arg or GAA increased (P < 0.05) feed efficiency from d 15 to 22 and d 8 to 22 posthatch. Birds fed the PC diet had greater (P < 0.05) responses for nearly all blood and tissue outcomes compared with NC-fed birds. Serum GAA was more responsive to supplementation of GAA in the presence versus absence of supplemental Arg (interaction, P < 0.001). Interactions (P < 0.05) were also observed for concentrations of muscle total Cr, creatinine, and most serum essential amino acids, notably Arg. Serum Cr, as well as muscle PCr, total Cr, and glycogen were increased (P < 0.05) independently by Arg and GAA supplementation, with highest levels achieved via combined addition of 0.12% GAA and 0.16% Arg. Minimal effects were detected on hematological and clinical chemistry outcomes. Overall, we conclude that GAA supplementation can spare Arg in broiler chicks fed Arg-deficient practical diets as evidenced by improvements in growth performance and muscle energy stores.