Two experiments were carried out to evaluate the effects of betaine (BET) supplementation in diets with reduced net energy (NE) levels on growth performance, nutrient digestibility and serum metabolomic profiles in growing pigs. In Exp. 1, 24 growing pigs (initial body weight, BW, 30.83 ± 2.50 kg) were allotted to one of the four treatments (six replications with one pig per pen) in a 2 × 2 factorial arrangement, including two dietary NE levels (2475 [N-NE] or 2395 [R80-NE] kcal/kg) and two BET doses (0 or 1500 mg/kg). In Exp. 2, 72 growing pigs were used in a 2 × 3 factorial arrangement, including three dietary NE levels (2475 [N-NE], 2415 [R60-NE] or 2355 [R120-NE] kcal/kg) and two BET doses (0 or 1500 mg/kg). Pigs with initial BW of 31.44 ± 1.65 kg were divided to one of the six treatments (six replications with two pigs per pen). In Exp. 1, lowing NE concentrations increased average daily feed intake (ADFI) by 10.69% in pigs fed the diet without BET (P > 0.05). BET significant increased ADFI in N-NE diet (P < 0.05) but had no influence on ADFI in R80-NE diet (P > 0.05). BET enhanced the apparent digestibility of crude protein (CP), dry matter (DM), organic matter (OM), gross energy (GE) and ether extract (EE) in R80-NE diet (P < 0.05). In Exp. 2, lowing NE concentrations enhanced ADFI (P > 0.05) and decreased average daily gain (ADG) (P < 0.05). The reduction in feed intake by BET was further enhanced as NE concentrations decreased from 2415 to 2355 kcal/kg (P < 0.10). BET reversed the elevation of serum triglyceride, alkaline phosphatase, aspartate aminotransferase and alanine aminotransferase levels caused by R120-NE diet (P < 0.05). The concentrations of cholecystokinin and glucagon-like peptide 1 were increased by BET in pigs fed R120-NE diet (P < 0.05). Serum metabolomics reveals that lowing dietary NE concentrations affected mainly amino acid biosynthetic pathways (P < 0.05). BET supplementation in R120-NE diet up-regulated serum BET levels and down-regulated homocysteine, DL-carnitine and four amino acid secondary metabolites (P < 0.05). In conclusion, lowing dietary NE contents reduced the growth performance and caused metabolic abnormalities in growing pigs. However, BET decreased feed intake to a certain extent and improved the metabolic health of pigs fed low-NE diets, which may be related to the dual regulation of amino acid metabolism and the secretion of appetite related hormones by BET.