Tridentate chelating ligands, which possess three coordination sites and six bonding electrons to coordinate a metal center, are considered as an alternative to the cyclopentadienyl ligand. Such tridentate ligands can stabilize a variety of metal oxidation states of a complex and form two four-, five-, six- or seven-membered chelate rings. In addition, fine tuning of its electronic and steric properties by the modification of coordination atoms, ligand skeleton, and substituents on the skeleton of the tridentate ligand will enable control of the catalytic activity and stereochemistry of its complexes in olefin polymerization. This review introduces the recent progress on transition metal catalysts bearing tridentate chelating ligands for olefin polymerization. Special emphasis is placed on the effects of ligand modifications on the polymerization activity, the stereochemistry, comonomer incorporation, and molecular weight of the resulting polymers.