A modified conjugated diene-based polymer comprising: a repeating unit derived from a conjugated diene-based monomer; and a functional group derived from a modifier, and satisfying the following conditions of (i) to (vii): (i) the molecular weight distribution curve by gel permeation chromatography is unimodal; (ii) molecular weight distribution is 2.0 or more; (iii) the weight average molecular weight is 100,000 g/mol to 2,000,000 g/mol; (vi) the modification ratio is 70% or more; (v) the mooney stress relaxation ratio measured at 100°C is 0.7 to 3.0; (vi) the mooney large relaxation area (MLRA) measured at 100°C and represented by the following Mathematical Formula 1 is 1500 MU-s or less; and (vii) nitrogen atom content is 100 ppm or more based on a weight of the polymer: A = k a + 1 t ƒ a + 1 − t o a + 1 in Mathematical Formula 1, A is a mooney large relaxation area (MLRA), k is a mooney intercept after 1 second from stopping operation of a rotor of a mooney viscometer, a is a slope of the log graph on change in mooney torque over time, t o is an initiation point of mooney relaxation, and t f is a finishing point of mooney relaxation, wherein the weight average molecular weight and the molecular weight distribution are measured through gel permeation chromatography as disclosed in the specification, the modification ratio and the mooney stress relaxation ratio are measured as disclosed in the specification. (i) the molecular weight distribution curve by gel permeation chromatography is unimodal; (ii) molecular weight distribution is 2.0 or more; (iii) the weight average molecular weight is 100,000 g/mol to 2,000,000 g/mol; (vi) the modification ratio is 70% or more; (v) the mooney stress relaxation ratio measured at 100°C is 0.7 to 3.0; (vi) the mooney large relaxation area (MLRA) measured at 100°C and represented by the following Mathematical Formula 1 is 1500 MU-s or less; and (vii) nitrogen atom content is 100 ppm or more based on a weight of the polymer: A = k a + 1 t ƒ a + 1 − t o a + 1 in Mathematical Formula 1, A is a mooney large relaxation area (MLRA), k is a mooney intercept after 1 second from stopping operation of a rotor of a mooney viscometer, a is a slope of the log graph on change in mooney torque over time, t o is an initiation point of mooney relaxation, and t f is a finishing point of mooney relaxation, wherein the weight average molecular weight and the molecular weight distribution are measured through gel permeation chromatography as disclosed in the specification, the modification ratio and the mooney stress relaxation ratio are measured as disclosed in the specification. A is a mooney large relaxation area (MLRA), k is a mooney intercept after 1 second from stopping operation of a rotor of a mooney viscometer, a is a slope of the log graph on change in mooney torque over time, t o is an initiation point of mooney relaxation, and t f is a finishing point of mooney relaxation, wherein the weight average molecular weight and the molecular weight distribution are measured through gel permeation chromatography as disclosed in the specification, the modification ratio and the mooney stress relaxation ratio are measured as disclosed in the specification.