A scallop (pronounced /ˈskɒləp/ or /ˈskæləp/) is a marine bivalve mollusk of the family Pectinidae. Scallops constitute a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source. Some scallops are valued for their brightly colored shells.

Like the true oysters (family Ostreidae), scallops have a central adductor muscle, and thus the inside of their shells has a characteristic central scar, marking the point of attachment for this muscle. The adductor muscle of scallops is larger and more developed than that of oysters, because they are active swimmers; scallops are in fact the only migratory bivalve. Their shell shape tends to be highly regular, recalling one archetypal form of a seashell, and because of this pleasing geometric shape, the scallop shell is a common decorative motif.

Most scallops are free-living, but some species can attach to a substrate by a structure called a byssus, or even be cemented to their substrate as adults (e.g. Hinnites spp.). A free-living scallop can swim, by rapidly opening and closing its shell. This method of locomotion is also a defense technique, protecting it from threatening predators. There can sometimes be an audible soft popping sound that some scallops make as they flap their shells underwater, and because of this, one kind was dubbed "singing scallops" by a seafood vendor.

Scallops are hermaphroditic; capable of switching sexes. Both sexes produce roe, whose coloring depends upon the parent's (current) sex. Red roe is that of a female, and white, that of a male. Spermatozoa and ova are released freely into the water during mating season and fertilized ova sink to the bottom. After several weeks, the immature scallop hatches and the larvae drift until settling to the bottom again to grow. They reach sexual maturity after several years, though they may not reach a commercially harvestable size until six to eight years of age. Scallops may live up to 18 years, with their age reflected in the annuli, the concentric rings of their shells.