Provided are a halogenated butyl rubber composition which can be made low in hardness, being excellent in damping property and having excellent balance of the physical properties among kneading, roll-processing and crosslinking, and a vibration-insulating grommet using the halogenated butyl rubber composition. The halogenated butyl rubber composition contains 100 parts mass of halogenated butyl rubber and 3 to 20 parts mass of liquid unsaturated polybutadiene, and preferably further contains 0.1 to 5 parts mass of fatty acid amide. The halogenated butyl rubber composition is suitable for producing a vibration-insulating grommet.