Letterman scored a weeklong average of 3.46 million viewers, edging out the 3.32 million viewers who watched NBC's "Tonight Show," hosted by Conan O'Brien.
It was Letterman's first win during a full week of all-new broadcasts since December 2005, when Oprah Winfrey was his guest on the CBS late-night show. Letterman was perhaps given a ratings boost from lingering controversy over a wisecrack that targeted the family of Alaska Gov. Sarah Palin.
O'Brien took over "Tonight" from longtime host Jay Leno earlier this month.