In a large skillet over medium-high heat, melt butter. When foaming subsides, add ham and cook, turning once, until well browned on both sides, about 8 minutes total. Transfer ham to a plate and reduce heat to medium.
Pour orange juice into skillet and, with a wooden spoon, scrape up any browned bits on bottom of skillet. Stir in mustard and cook until liquid thickens slightly, about 3 minutes. Add pineapple and cook 2 minutes longer, then return ham to skillet and turn to coat with sauce.