To provide a rubber composition improved well balancedly in low fuel consumption, wet grip performance and dry grip performance, and to provide a pneumatic tire using the same.

The rubber composition contains a rubber component and a silica. In 100 mass% of the rubber component, the content of a conjugated dienic polymer is 5 mass% which has a structural unit based on a conjugated diene and a structural unit represented by formula (I), and at least one terminal of which is modified with a compound having a carbonyl group. When one particle having ≥3 adjacent particles is taken as a branch particle Z, the average length W¹ between branch particles Z-Z including branch particles Z in the silica is 30 to 400 nm. The content of the silica is 5 to 150 pts.mass based on 100 pts.mass of the rubber component. In the formula (I), X¹, X² and X³ are each independently a group represented by formula (Ia), a hydroxyl group, a hydrocarbyl group, or a substituted hydrocarbyl group; and at least one of X¹, X² and X³ is a group represented by formula (Ia). In the formula (Ia), R¹ an R² are each independently a 1-6C hydrocarbyl group, a 1-6C substituted hydrocarbyl group, a silyl group, or a substituted silyl group, and R¹ an R² may bond to form a ring structure with a nitrogen atom.