At 55.56 percent, Denmark was close behind Sweden. Spain, the Netherlands, and Finland rounded out the top five.
Both Finland and Spain, as well as Slovenia, were the biggest movers in this year's report, which saw a total of nine countries raising taxes for high income earners, and four lowering them.
The consultancy found that the global financial crisis had an impact on tax rates in a number of countries.
"Many countries need to consolidate their public finances in the wake of the financial crisis and increasing taxes for high income earners is considered necessary," Tina Zetterlund, head of the tax department at KPMG in Sweden, said in a statement.
"Even if the increases often give limited revenue to the state coffers, they're seen as being politically necessary to gain acceptance for benefit reductions that hit low-income earners hard."
In the report, a marginal tax rate is defined as the tax high-earners pay on their last unit of income. In Sweden's case, that means high-earners pay 56.6 percent in tax on their last earned krona.
The KPMG data compares tax rates in over 100 countries, including the 33 industrialized countries of the OECD. Hungary pulled in the lowest spot of the OECD countries, with a highest marginal tax rate of just 16 percent.