Conducted by the Catalan regional government's CEO survey institute, the figures showed 41.9 percent were in favour of seceding from Spain, down from 44.0 percent in July.
At the same time, the number of those who want the wealthy northeastern region to remain part of Spain rose slightly to 48.8 percent from 48.3 percent in the previous poll.
The figures were based on the responses of 1,315 residents questioned between September 16 and October 7 as tensions were rising ahead of the anticipated Supreme Court ruling.
On October 14, the court issued its explosive verdict, handing heavy jail terms to nine separatist leaders over an abortive 2017 independence bid, setting off a wave of angry protests that repeatedly descended into violence.
Catalonia's parliament and its regional government are dominated by separatist parties, but the region itself remains deeply divided over the matter, and the recent crisis has exacerbated the split.