The present invention pertains to: a modified conjugated diene-based polymer having excellent workability as well as excellent tensile strength and viscoelastic properties; and a rubber composition containing same. Provided is a modified conjugated diene-based polymer comprising: a first chain containing a repeating unit derived from a conjugated diene-based monomer; a second chain containing a repeating unit derived from a compound represented by chemical formula 1; and a unit derived from an alkoxysilane-based modifier, wherein the first chain and the second chain are coupled by the modifier-derived unit.