A positive active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same are disclosed, and the positive active material for a rechargeable lithium battery includes: a layered lithium composite oxide core; a first coating layer on the surface of the core and magnesium doped at a lithium site in the core; a NiO-phase second coating layer on the first coating layer wherein the NiO-phase has a rock salt structure; and a third coating layer on the second coating layer and including lithium magnesium phosphate.