Private sector business activity increased at the weakest rate for 20 months, according to a new survey.
The loss of momentum reflects a drop in new work, the Bank of Scotland said.
Its research shows only a marginal increase in total activity across manufacturing and services, below the UK-wide average.
Donald MacRae, chief economist at the bank, said there was 20 months of growth.
"However, manufacturing output fell and new domestic and export orders decreased, although employment continued to grow in the service sector," he said.
"The Scottish economy is struggling to maintain growth momentum in this latest slowdown."
The amount of new work placed with Scottish businesses fell for the second month running in August.
The overall rate of decline accelerated from July to the fastest recorded since December 2010.
Five other UK regions saw reductions in new work but only Northern Ireland posted a more marked drop than Scotland.
Cost pressures rose during August, which anecdotal evidence links to increases in the cost of fuel and raw materials, the bank said.
Finance Secretary John Swinney said: "We are doing all we can within our current powers to strengthen the economy, to create and bring jobs to Scotland, to stimulate growth, and to create the most supportive environment for business in the UK.
"But it is clear that much more could be achieved and the UK Government's repeated failure to give the green light to much needed capital investment is hampering progress."
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