During the four weeks to the end of October, Scottish sales decreased by 6.8% compared with October 2019, when they had decreased by 0.8%.
The figures also revealed a three-month average decline of 7.2% and 12-month average decrease of 9.4% for retail sales.
Food sales, however, have shown their highest growth since March, up by 4.4% against October last year.
The October performance was better than the three-month average growth of 3.2% and the 12-month average of 3.5%.
David Lonsdale, director of the Scottish Retail Consortium, said: “Retail sales in Scotland have still to emerge from the pandemic-induced funk, after eight months, deeply concerning at what is for many stores the start of the all-important festive period.
“Convenience store sales fared well, temporarily buoyed as restrictions on eating out became more prevalent.”