Rumsfeld, who was U.S. defense secretary under President George W. Bush, posted a picture on Twitter of the letter he sent to the Internal Revenue Service, saying he had submitted his tax return by Tuesday's deadline but that he had "absolutely no idea whether our tax returns and tax payments are accurate."
That was despite his being a college graduate and having paid an accounting firm to prepare his return, he said.
Rumsfeld had a key role in the U.S. invasion of Iraq in 2003 and in the buildup to that war assessed intelligence on Baghdad's supply of weapons of mass destruction by creating three categories - known knowns, unknown unknowns and known unknowns.
Rumsfeld's tax records fall into the third category - something he knows he doesn't know about - and in his letter he blamed the U.S. tax code.
"The tax code is so complex and the forms so complicated that I know that I cannot have any confidence that I know what is being requested and therefore I cannot and do not know, and I suspect a great many Americans cannot know, whether or not their tax returns are accurate," he wrote.
"I do hope that at some point in my lifetime, and I am now in my 80s, so there are not many years left, the U.S. government will simplify the U.S. tax code so that those citizens who sincerely want to pay what they should, are able to do it right and know that they have done it right."
Rumsfeld pointed out that his wife, another college graduate, also has no idea if the tax payments were correct.