A novel type of magnetically responsive polymer nanocomposite Fe₃O₄@poly(undecylenic acid-co-4-vinyl pyridine-co-sodium acrylate) (Fe₃O₄@PUVS) was synthesized by the free radical polymerization of 4-vinyl pyridine (4-VP) with sodium acrylate (SAA) and Fe₃O₄@undecylenic acid. Pd²⁺ was then immobilized on this magnetic nanocomposite to form the magnetic Fe₃O₄@PUVS-Pd catalyst. This catalyst exhibited excellent catalytic activity for the Heck and Suzuki coupling reactions in water, and could be simply separated by using a permanent magnet. The supported catalyst could be used consecutively for six runs without significant loss of catalytic activity.