Pectin, a complex class of plant polysaccharides, is composed of a galacturonan backbone and neutral sugar side chains. Natural pectin is reported to prevent colon cancer as a dietary fiber (DF). To enhance its bioavailability and bioactivity, pectin was modified into bioavailable modified pectin fragments (MPs) with low molecular mass. Also, MPs had low degrees of esterification (DE) which is reported to inhibit tumor growth, induce apoptosis, suppress metastasis, and modulate immunological responses. Antitumor activity of MPs chiefly arises from intervention in ligand recognition by galectin-3 (Gal-3). In addition, pectin is a suitable vehicle for anti-cancer drug delivery systems, due to its abundant modifiable functional groups and special physicochemical properties. Here, we summarize the structural features, bio-absorption and antitumor mechanisms and the structure-activity relationship of MPs. We also offer prospects and challenges for developing pectin into nutraceuticals or drugs.