The present invention provides rubber compositions and tires which provide improved overall performance in terms of wet grip performance and dry grip performance. Included is a rubber composition having a hardness that reversibly changes with water, satisfying the relationships (1) and (2) below, and having a stress at break of 8.0 MPa or more, Hardness when dry−Hardness when water-wet≥1  (1) wherein each hardness represents the JIS-A hardness at 25° C. of the rubber composition, 0.90≤Volume when water-wet/Volume when dry≤1.01  (2) wherein each volume represents the volume at 25° C. of the rubber composition.