They reveal a candidate adeptly tailoring his message to a federal audience and tapping into many workers' dismay at funding and staffing cuts in the Bush years, the Post said.
The letters describe Obama's intention to scale back on contracts to private firms, remove censorship from scientific research and seek tougher regulation to protect federal workers and the environment.
Obama, a Democratic senator from Illinois, championed change in his campaign for the White House that ended in victory over Republican John McCain.
Obama will take office as the first black U.S. president on January 20, backed by public support for a new direction but burdened by the global financial crisis, a weak U.S. economy and wars in Iraq and Afghanistan.
Before the election, Obama wrote to employees in the Departments of Labor, Defense, Housing and Urban Development (HUD) and Veterans Affairs, the Transportation Security Administration, the Environmental Protection Agency and the Social Security Administration, the Post said.
John Gage, president of the 600,000-member American Federation of Government Employees, requested that Obama write the letters, which were distributed through the union.
"The fact that he's willing to put his name to it is a good sign," Gage told the Post.
The paper said Obama echoed in his letters a long-standing lament of federal workers -- that the Bush administration starved their agencies of staff and money to the point where they could not do their jobs.
Obama also made clear that HUD would have an enhanced role in restoring public confidence in the U.S. housing market, where the global credit crisis had its roots.
Obama insisted that "HUD must be part of the solution" to the housing crisis and to keeping an estimated 5.4 million more families from losing homes in foreclosures.