​The present study investigated the effects of supplementing broiler chicken diets with green seaweed Ulva lactuca (SW), an enzyme mixture (Enz), or their combination (SW+Enz) on growth performance, immune index, intestinal morphology, nutrients digestibility, cecal microbiota balance, and cecal short chain fatty acid (SCFA) concentrations. A total of 960 one-day-old male Ross 308 broiler chickens were reared under standard management practices for 35 days. Four dietary treatments were randomly assigned across 12 replicates per treatment (20 chicks each). The treatments included: (C) a control diet (basal); (Enz) control diet supplemented with Enz (250 mg/kg); (SW) a SW diet (10 g/kg); and (SW+Enz) SW+ Enz combination (at the aforementioned doses). Results showed that all supplemented groups significantly improved body weight gain and feed conversion ratio (FCR) (P < 0.05) compared to the control, with the SW+Enz group achieving the highest values. Specifically, FCR improved by 7.4 % (P < 0.05) with individual SW or Enz supplementation, and by 12.3 % (P < 0.05) with the SW+Enz combination, all relative to the control diet. Supplementation with either Enz or SW alone enhanced (P < 0.05) the digestibility coefficients for crude protein and gross energy (GE). Additionally, these individual treatments led to an increased retention (P < 0.05) of AME and AMEn when compared to the control group. The combination of SW+Enz led to significantly greater (P < 0.05) improvements in the digestibility coefficients of crude fat, starch, GE, NDF, ADF, AME, and AMEn when compared with individual inclusion. Relative to the control, single SW inclusion significantly enhanced energy retention by 4 % and protein retention by 6 % (P < 0.05). The synergistic effect of combined SW+ENZ further improved energy retention by 9.1 % and protein retention by 11.7 % (P < 0.05). The SW+Enz combination demonstrated significant improvements (P < 0.05) across several parameters, including the immune index, intestinal villus height, and villus surface area, surpassing other treatment groups. Furthermore, this combination resulted in higher counts of beneficial cecal bacteria and increased SCFA concentrations (P < 0.05) compared to supplementing with SW alone. These findings highlight SW+Enz as a promising sustainable feed additive for poultry production, aligning with growing interest in alternatives that enhance productivity while reducing reliance on antibiotics and synthetic growth promoters.​​