This study was conducted to investigate the effect of anhydrous betaine and hydrochloride betaine on growth performance, meat quality, relaxometry, postmortem glycolysis, and antioxidant capacity of partridge shank broiler chickens. A total of 400 one-day-old male broilers were randomly divided into 5 treatments and fed basal diets supplemented with 0 (control), 500 (L-AB) or 1,000 (H-AB) mg/kg anhydrous betaine, and 642.23 (L-HB) or 1,284.46 (H-HB) mg/kg hydrochloride betaine, respectively. Compared with the control group, anhydrous betaine supplementation significantly increased (P < 0.05) average daily gain and decreased (P < 0.05) drip loss in breast and thigh muscles of broilers. The H-AB group further increased (P < 0.05) breast muscle yield, pH, immobile water proportion (P), the contents of crude protein and glutathione (GSH), the activities of creatine kinase (CK) and glutathione peroxidase (GPX), the mRNA expressions of glucose transporter 4 (GLUT4), protein kinase AMP-activated non-catalytic subunit gamma 3 (PRKAG3), nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase 1 (HO-1) in breast muscle, and a*, GLUT4 mRNA expression in thigh muscle, and decreased (P < 0.05) drip loss, free water proportion (P), the contents of lactate and malondialdehyde (MDA) in breast muscle. Moreover, the H-HB group significantly increased (P < 0.05) pH, P proportion, the activities of CK, total antioxidant capacity (T-AOC), total superoxide dismutase (T-SOD), and GPX, the content of GSH, the mRNA levels of Nrf2, HO-1, GPX, and γ-glutamate-cysteine ligase catalytic subunit (γ-GCLc) in breast muscle, and the activity and mRNA expression of GPX in thigh muscle, and decreased (P < 0.05) drip loss, P proportion in breast muscle, and MDA content in breast and thigh muscles. In conclusion, anhydrous betaine showed better effects than hydrochloride betaine in improving growth performance and breast muscle yield of broilers. Moreover, anhydrous betaine (1,000 mg/kg) or equimolar hydrochloride betaine supplementation could improve meat quality by decreasing drip loss, free water proportion, and lactate content, and enhancing muscle antioxidant capacity.