The adhesive wear of steel is a crucial issue in many industrial fields because it can lead to serious machine failure. However, the adhesive wear mechanism is still under debate owing to its complexity. Therefore, in this work, we performed reactive molecular dynamics-based sliding simulations of single crystalline body-centered cubic iron and investigated the fundamental atomic-scale adhesive wear mechanism for improving the wear resistance of steel. The effects of surface orientation, sliding direction, and humid atmosphere on the adhesive wear property were analyzed. In the sliding simulation, we observed two adhesive wear types. One is the wear accompanying surface deformation, in which the surface asperities gradually deform by slip and adhere severely. The other is the wear accompanying surface fracture with crack generation. The former can lead to seizures, whereas the latter can lead to wear debris formation. We propose that the rubbing surface orientation and sliding direction alter the atomic-scale adhesive wear type. Wear with surface deformation occurred when the deformation by slip was favorable, whereas wear with surface fracture occurred when slip was not favorable. Understanding the adhesive wear mechanism of iron in humid atmospheres is also important in many industrial fields. When water molecules were present at the sliding interface, both types of adhesive wear were suppressed. At the sliding interface, Fe–OH and Fe–O–Fe groups were formed on the scars through the tribochemical reaction with water. These groups passivated the nascent Fe surfaces and suppressed adhesion to the counter surface, thereby reducing adhesive wear. Therefore, we conclude that the surface orientation and sliding direction determine the atomic-scale adhesive wear type, whereas a humid atmosphere affects the wear amount at the atomic scale.