The rating agency, whose move mirrored last week's downgrade of Spain by Fitch, cited Spain's high unemployment, tightening credit and high private-sector debt among reasons for cutting the nation's long-term rating to AA- from AA.
S&P announced the downgrade as finance ministers and central bank chiefs from the world's 20 biggest economies were due to meet later Friday in Paris to find an urgent and convincing solution to the deepening debt crisis.
"Despite signs of resilience in economic performance during 2011, we see heightened risks to Spain's growth prospects due to high unemployment, tighter financial conditions, the still high level of private sector debt, and the likely economic slowdown in Spain's main trading partners," S&P said.
It also noted the "incomplete state" of labour market reform and the likelihood of further asset deterioration for Spain's banks, and it downgraded its forecast for Spanish economic growth in 2012 to about 1 per cent.
In February, S&P had forecast 1.5 per cent growth for 2012.
Like Fitch, which also now rates Spain at AA-, S&P signalled further possible downgrades for Spain, saying there was still a risk the euro zone's fourth-largest economy could slip into recession next year, with a 0.5 per cent contraction.
"We could lower the ratings again if, consistent with our downside scenario, the economy contracts in 2012, Spain's fiscal position significantly deviates from the government's budgetary targets, or additional labour market and other growth-enhancing reforms are delayed," S&P said.
The euro dipped in Asian trade after the downgrade, though it still remained on track for its