The present invention provides a catalyst composition including a functionalizing agent of the following Formula 1 together with a rare earth metal compound, an alkylating agent, and a halogen compound, having good catalytic activity and polymerization reactivity and useful for the preparation of a conjugated diene-based polymer having high linearity and excellent processability, and a conjugated diene-based polymer prepared using the catalyst composition. (X1)a—Sn—(X2)4-a,  [Formula 1] In Formula 1, a, X1, and X2 are the same as defined in the disclosure.