Total sales were 1.8% lower in July than they were a year ago, with like-for-like sales - which strip out factors such as new store openings - down by 3.7%.
However, experts said the numbers are not as bad as the figures suggest given July 2013 saw an increase of 4% in total sales - which was the strongest growth recorded all year.
Total food sales were 2.8% lower last month, the deepest decline since March, the latest Scottish Retail Sales Monitor report said.
David Lonsdale, director of the Scottish Retail Consortium, said: “Despite a healthy rise in footfall witnessed on Scotland’s high streets and shopping centres in July, it didn’t translate into a commensurate increase in the actual value of retail sales.
“The total value of sales grew by a meagre 0.1% in real terms, once shop price deflation is taken into account.
“The bright spot once again was sales of non-food items which, adjusted for the contribution of online retailing, rose by 0.7%, its strongest growth over the past three months.”
He said that rise was driven by Commonwealth Games-related purchases as well as strong trade in outdoor furniture, DIY and home furnishings.
Mr Lonsdale added: “Further discounting across the grocery sector led to a fall in the total value of food sales, however comparisons with last year ought to be tempered as July 2013 was a high water mark for food-related sales.
“A number of high-profile sporting successes in the same period last year led to bumper demand for celebratory food and drink.”
David McCorquodale, head of retail at KPMG, said: “Against a very tough set of comparables last year, when we were basking in a heatwave and a Scottish Wimbledon champion, the July sales numbers in Scotland are not as bad as the headlines perhaps suggest.
“Clothing and footwear have enjoyed a better spring and early summer this year, and so it is not surprising they have cooled off relatively in the last couple of months against strong figures last year.
“The decline in other non-food is not as sharp as it was earlier in the year as homeowners feel a little more confident to spend on furniture, albeit the confidence is cautious.
“Scotland’s high streets saw improved footfall in July, no doubt boosted by Glasgow’s Commonwealth Games.
“However, as with the Olympics two years ago, these events may boost the restaurant trade but don’t always boost high street sales.”