Provided are an additive for skim coat mortar and a skim coat mortar composition including the same, and the additive is a blend of cellulose ether having hydroxyalkylalkyl cellulose cross-linked with an aldehyde compound and hydroxyalkyl cellulose cross-linked with an aldehyde compound. By applying the additive to a skim coat mortar composition, it is possible to improve workability, surface luster, and a creamy property while maintaining a water retention property.