In the present study, a hot water crude extract from Ulva intestinalis (Ui-HWCE) was used as a dietary supplement, and the effects on growth, immune responses, and resistance against white spot syndrome virus (WSSV) and yellowhead virus (YHV) infection in Pacific white shrimp (Litopenaeus vannamei) were investigated. Chemical analyses of Ui-HWCE revealed 13.75 ± 0.41% sulfate, 37.86 ± 5.96% uronic acid, and 46.63 ± 5.16% carbohydrate contents. The monosaccharide content of Ui-HWCE contained glucose (6.81 ± 0.94%), xylose (4.15 ± 0.11%), and rhamnose (25.84 ± 0.80%). Functional group analysis of Ui-HWCE by Fourier transform infrared (FTIR) spectroscopy revealed a typical infrared spectrum of ulvan similar to the infrared spectrum of commercially purified ulvan from Ulva armoricana (77.86 ± 2.19% similarity). Ui-HWCE was added to shrimp diets via top-dressing at 0, 1, 5, and 10 g/kg diet. After 28 days, Ui-HWCE supplementation at 5 g/kg diet efficiently improved shrimp growth performance, as indicated by weight gain, average daily growth, specific growth rates, and villus height determined by observing gut morphology. Additionally, Ui-HWCE feed supplementation at 5 g/kg diet significantly increased immune responses against a pathogenic bacterium (Vibrio parahaemolyticus AHPND stain), including phagocytic activity and clearance efficiency. Furthermore, Ui-HWCE feed supplementation upregulated the expression of several immune-related genes in the hemocytes and gills. Ui-HWCE supplementation at 1 and 5 g/kg resulted in effective anti-YHV but not anti-WSSV activity, which significantly decreased the mortality rate and YHV burden in surviving shrimp. It was concluded that Ui-HWCE supplied at 5 g/kg diet exhibits growth-promoting, immune-stimulatory, and antiviral activity that could protect L. vannamei against YHV infection.