As a seaweed supplement, the powdered, entire herb of Asparagopsis taxiformis can effectively promote immune response in animals. However, the active factors responsible for this effect are currently unknown. A novel polysaccharide, ATP-1, was isolated and purified from the red alga A. taxiformis, and its structural and immunomodulatory properties were determined. ATP-1 is a neutral polysaccharide with a molecular weight of 307.978 kDa and is primarily composed of galactose (94.07%), glucose (1.27%), and xylose (4.66%). Scanning electron microscopy and atomic force microscopy revealed that ATP1 had an amorphous structure and was composed of molecular chain entanglement and intramolecular hydrogen bonding interactions. The microstructure of ATP-1 exhibited asymmetric sheet–like cracks and pores. Results from immunomodulatory activity showed that ATP-1 promoted inducible nitric oxide (NO) synthase and cyclooxygenase (COX)-2 protein expression in RAW264.7 macrophages, improved macrophage phagocytic ability, and stimulated cytokine production (NO, tumor necrosis factor (TNF)-α, and interleukin (IL)-6). Quantitative proteome analysis identified 178 differentially expressed proteins (70 upregulated and 108 downregulated) that were involved in various molecular pathways, among which, TNF was one of the 20 most common and significant signaling pathways in inflammation and immunoregulation. Western blotting indicated that ATP-1 significantly upregulated the expression of IKKα, IKKβ, COX-2, and TNF-α proteins in the TNF pathway. In contrast, ATP-1 downregulated the expression of IκBα, Fas, and AIP1. Results from molecular docking suggested that hydrogen bonding predominantly contributed to the affinity between TNF-α and galactose, glucose, and xylose. These findings indicated that ATP-1 may play an immunomodulatory role by activating the TNF signaling pathway. ATP-1 enhanced macrophage phagocytic capacity and stimulated the production of cytokines NO, TNF-α, and IL-6, thereby exerting immunoregulatory effects. Our findings highlight the potential of ATP-1 as a natural immune modulator with applications as a functional food.