A modified conjugated diene-based polymer is provided, having remarkable processability and excellent tensile strength and viscoelasticity properties, and a rubber composition including the same is also provided. The modified conjugated diene-based polymer has a unimodal shape molecular weight distribution curve by gel permeation chromatography, narrow molecular weight distribution and a Mooney stress-relaxation ratio of 0.7 or more, and includes a functional group derived from a modifier represented by Formula 1, and thus has remarkable processability and excellent tensile properties and viscoelasticity properties.