Background Molasses is a potential energy supplement; extensively used to improve growth performance, milk and meat characteristics in goats at relatively low concentrations of 5–40% of the diet. Few data are available concerning feeding molasses to goat kids; therefore, the current study aimed to investigate the effects of dietary supplementation with higher concentrations of molasses on growth performance, blood metabolites and rumen fermentation indices. Twenty male Nubian goat kids (4–6 months old; 9–10 kg BW) were randomly assigned to 4 groups receiving different concentration of molasses: 0% (M-0), 30% (M-30), 40% (M-40) and 45% (M-45) for 5 weeks. Feed (DFI) and water intake (DWI) were measured daily, while the blood and rumen liquor samples were collected weekly. Results The DFI increased and feed conversion ratio (FCR) decreased in all molasses-supplemented groups (P ≤ 0.05), whereas DWI increased in M-30 and decreased in M-45 (P ≤ 0.05). The final BW and average daily gain (ADG) increased (P < 0.0001) in groups M-30 and M-40 compared to the control and M-45. Blood pH was significantly influenced by dietary molasses concentration (MC) and the duration of molasses supplementation (MD), where it decreased in groups M-30 and M-45 compared to the control and M-40 (P < 0.05). The MC had no significant effect on blood Hb, HCT, TLC, albumin, [K+], AST, ALT and total protozoa count (TPC), as well as ruminal-[Na+], [K+], strong ion difference concentration ([SID3]) and [NH3]; however, only [NH3] was significantly affected by MD and the interaction between MC and MD (MC × MD). Serum TP, globulins, [Na+] and [Cl−] increased (P ≤ 0.05) in all supplemented groups, while A/G ratio and [SID3] decreased (P ≤ 0.05). Ruminal pH decreased (P < 0.0001) in M-40 and M-45 compared to the control and M-30. However, [VFAs] increased (P < 0.04) in M-30 and M-40 compared to the control and M-45, while osmolality increased (P ≤ 0.05) in M-30 compared to the other groups. Conclusions Dietary supplementation with molasses at a concentration of 30% for 3 weeks improved growth performance, protein metabolism and rumen fermentation without compromising animal health, immunity, and electrolytes and acid-base homeostasis.