The 20-city index fell 3.8 percent from a year earlier, while unadjusted prices fell 3.9 percent in a 10-city index.
For both the 10-city and 20-city indexes, it was the ninth consecutive monthly decline. From December to January, both composites fell 0.8 percent, the report said.
Of the 20 cities monitored, prices from December rose in only Miami, Phoenix and Washington. Prices were not listed for Charlotte, N.C., due to delays in data reporting.
Looking for a silver lining in the report, S&P/Case-Shiller said the annual rate of decline in December was 4.1 percent for both the 10-city and 20-city indexes. On the negative side, Atlanta in January posted the lowest annual return at minus 14.8 percent.
Denver, Detroit and Phoenix were the only cities with annual growth rates with prices rising 0.2 percent, 1.7 percent and 1.3 percent, respectively.
"Despite some positive economic signs, home prices continued to drop. The 10- and 20- City Composites and eight cities -- Atlanta, Chicago, Cleveland, Las Vegas, New York, Portland, Seattle and Tampa -- made new lows," said David Blitzer, chairman of the Index Committee at S&P Indexes.
"Atlanta continues to stand out in terms of recent relative weakness," he said. "It was down 2.1 percent over the month, and has fallen by a cumulative 19.7 percent over the last six months," Blitzer noted.
"Seven of the cities were down by 1 percent or more over the month. With the new lows, both composites are now 34.4 percent off their relative 2006 peaks," he said.