互补品（Complement/Complement good）乃指商品间的消费是合并在一起，相互之间彼此牵动对方的需求量。如果一种商品价格提高，会导致另一种商品的需求下降。

左鞋和右鞋是互补品。如果没有左鞋，无论拥有多少右鞋，效用也是相同的。因此，互补品的无差别曲线成直角形。

烤肉的时候木炭与肉的关系，因为用木炭烤肉才会比较香，若是肉买多出一般的量，为了怕肉不熟相对的木炭也会买多一点；又比如说羽毛球拍和羽毛球的关系：由于此两件物品乃同时存在才能发挥应有之功效，当羽毛球拍的价格下降时，相对地人们也会因为希望拥有较好的羽毛球以供运动之用，故羽毛球的需求便有机会在其价格没有变动的可能下有所提升。

In economics, a complementary good or complement is a good with a negative cross elasticity of demand, in contrast to a substitute good. This means a good's demand is increased when the price of another good is decreased. Conversely, the demand for a good is decreased when the price of another good is increased. If goods A and B are complements, an increase in the price of A will result in a leftward movement along the demand curve of A and cause the demand curve for B to shift in; less of each good will be demanded. A decrease in price of A will result in a rightward movement along the demand curve of A and cause the demand curve B to shift outward; more of each good will be demanded. Basically this means that since the demand of one good is linked to the demand of another good, if a higher quantity is demanded of one good, a higher quantity will also be demanded of the other, and if a lower quantity is demanded of one good, a lower quantity will be demanded of the other. The prices of complementary goods are related in the same way: if the price of one good rises, so will the price of the other, and vice versa. With substitute goods, however, the price and quantity demanded of one good is related inversely to the price and quantity demanded of a substitute good, meaning that if the price or quantity demanded of one good rises, the price or quantity demanded of its substitute will fall.