The present invention provides a rubber composition for an undertread, which has improved in processability, fuel economy, and tensile properties in a balanced manner, and also provides a pneumatic tire formed from the rubber composition. The present invention relates to a rubber composition for an undertread, including: a rubber component containing a modified natural rubber having a phosphorus content of 200 ppm or less; and carbon black and/or a white filler, wherein the modified natural rubber is contained in an amount of 5% by mass or more per 100% by mass of the rubber component.