On a like-for-like basis, quarterly sales dropped by 2.8%, excluding fuel, and by 4.1%, including fuel.
Sainsbury chief executive Mike Coupe said: "The market remains dynamic and fiercely competitive. The long-running trend of more frequent, convenient shopping has accelerated, resulting in smaller basket sizes.
"An increase in price investment and short-term competitor promotional activity, combined with favourable commodity markets, has resulted in deflation in many areas of our food business."
"Our general merchandise and clothing businesses continue to deliver strong growth. Our clothing brand Tu celebrated its tenth anniversary during the quarter with the launch of our largest ever collection, which helped to drive double-digit sales growth. We also successfully launched an online clothing trial to selected customers in the Midlands.
During the 16-week period, the retailer opened 23 new convenience stores and refurbished 10 convenience stores.
"Our convenience business reached annualised sales of £2 billion and continues to grow strongly, at around 17 per cent."
During the first half, excluding fuel, total retail sales were flat and like-for-like sales were down by 2.1%, the company said.
The company attributed its second-quarter performance to the accelerated pace of change in the grocery market, including significant pricing activity and food price deflation in many areas.