A rubber composition for a tire for heavy loads of the present technology comprises: diene rubber containing at least 60% by mass of natural rubber; silica; carbon black; and a silane coupling agent. The silane coupling agent is polysiloxane represented by a specific average compositional formula. The content of the silica is from 5 to 50 parts by mass per 100 parts by mass of the diene rubber. The content of the carbon black is from 5 to 40 parts by mass per 100 parts by mass of the diene rubber. The total content of the silica and the carbon black (R) is from 30 to 70 parts by mass per 100 parts by mass of the diene rubber. The content of the silane coupling agent is from 2 to 20% by mass of the silica content.