"S&P Global Ratings placed its ratings on the Autonomous Community of Catalonia on CreditWatch with negative implications," the rating agency said in a statement released late on Wednesday.
"The Catalan government's political confrontation with Spain's central government has escalated following a referendum in Catalonia on October 1st on the region's independence," the statement said.
"We see a risk that this escalation may damage the coordination and communication between the two governments, which is essential to Catalonia's ability to service its debt on time and in full."
As a result, S&P said it would place Catalonia's ratings -- currently "B+/B" -- "on CreditWatch with negative implications."
The agency said it expected to "resolve the CreditWatch within the next three months."
In Spain's worst political crisis in decades, Catalonia held an independence referendum on Sunday, although the vote was banned by Madrid.
Images of police beating unarmed Catalans taking part in the vote sparked global concern.
Spain's key IBEX 35 stock index plunged by more than three percent Wednesday in the ongoing turbulence, with some big Catalan banks down more than five percent.