No, it isn’t.
Poverty comparisons across affluent nations typically use a “relative” measure of poverty. For each country the poverty line — the amount of income below which a household is defined as poor — is set at 50% (sometimes 60%) of that country’s median income. In a country with a high median, such as the United States, the poverty line thus will be comparatively high, making a high poverty rate more likely. Measured this way, the U.S. does indeed have the most poverty among the rich nations. That leads to statements such as Paul Krugman’s in his otherwise insightful op-ed in Monday’s New York Times: “Poverty rates are much lower in most European countries than in the United States.” (See also here and here.)
Though widely used, and not without merit, a relative measure should not be the principal basis for poverty comparisons. It focuses too heavily on the distribution of income and too little on the absolute income level of those at the bottom. Using a relative measure, the U.S. poverty rate is higher than Romania’s and only slightly lower than Mexico’s (see here). Similarly, Mississippi’s relative poverty rate is the same as Connecticut’s.