Sacha inchi soluble dietary fiber (SISDF) was evaluated for its antioxidant and immunological activity. The structure of SISDF was identified by ion chromatography, gel chromatography and Fourier transform infrared (FTIR) spectroscopy. The results showed that the weight-average molecular mass of SISDF was 401479 Da, which was found to be a large polysaccharide with pyranose as the main chain. The monosaccharide composition analysis showed that SISDF consisted of galacturonic acid (46.49%), rhamnose (18.95%), xylose (17.60%) and galactose (11.25%). At 2 mg/mL, SISDF scavenged 68.57% of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical, 69.83% of hydroxyl radical and 70.35% of 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation. At concentrations of 62.5–1000 μg/mL, SISDF enhanced the proliferative and phagocytic activities of RAW264.7 macrophages significantly (P < 0.01) and increased the secretion levels of NO, interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α factors in RAW264.7 macrophages. Therefore, sacha inchi soluble dietary fiber has good antioxidant and immune activity, and thus has the potential to be developed as a natural food antioxidant and anti-inflammatory nutraceutical.