Disclosed are a conjugated diene-based polymer from which a polymer composition excellent in fuel cost-saving properties and elongation at break can be obtained, a polymer composition containing the conjugated diene-based polymer and a reinforcing agent, and a process for producing the conjugated diene-based polymer. A conjugated diene-based polymer obtained by reacting one end of a polymer having a monomer unit based on a conjugated diene, a monomer unit based on a compound represented by the following formula (1), and a monomer unit based on a compound represented by the following formula (2) with an alkoxysilane compound. V1-S1   (1) wherein V1 represents a hydrocarbyl group having a polymerizable carbon-carbon double bond, and S1 represents a substituted silyl group. V2-A2   (2) wherein V2 represents a hydrocarbyl group having a polymerizable carbon-carbon double bond, and A2 represents a substituted amino group, or a nitrogen-containing heterocyclic group.