Disclosed are a transition metal precursor for preparing a lithium composite transition metal oxide, a method for preparing the precursor, and a lithium composite transition metal oxide. The transition metal precursor includes a composite transition metal compound having a composition represented by Formula (1) and a Mn content of 60 to 85 mol %: NiaMbMn1-(a+b)(OH1-x)2  (1) where M is at least one selected from the group consisting of Ti, Co, Al, Cu, Fe, Mg, B, Cr, Zr, Zn and period II transition metals, 0.15≤a≤0.3, 0≤b≤0.1 and 0