A cutting insert has a main body which is provided with an upper surface, a side surface, at least one groove which is formed in the side surface and which extends to the upper surface at one end, and a plurality of split cutting blades which are split by the at least one groove. The upper surface is provided with inclined portions corresponding to the split cutting blades. The distance of respective lower ends of the inclined portions from the corresponding split cutting blades increases toward the centers of the corresponding cutting blades in a plan view. A cutting tool having the cutting insert and a method for cutting a workpiece using the cutting tool are also provided.