The decision, which was expected, came a day after Armstrong announced he would no longer fight USADA claims that he used performance-enhancing drugs.
The USADA had said Thursday it would take away Armstrong's titles.
"Nobody wins when an athlete decides to cheat with dangerous performance-enhancing drugs, but clean athletes at every level expect those of us here on their behalf, to pursue the truth to ensure the win-at-all-cost culture does not permanently overtake fair, honest competition," USADA Chief Executive Officer Travis T. Tygart said.
"Any time we have overwhelming proof of doping, our mandate is to initiate the case through the process and see it to conclusion as was done in this case."
Earlier Friday, Union Cycliste Internationale, the international governing body of cycling, asked the USADA for a "reasoned decision explaining the action taken [against Armstrong]."
Armstrong said the USADA doesn't have the authority to take away his titles but Tygart told CBS that "given that he's chosen not to contest, it's a legal fact that he's now disqualified and has a permanent ban from sport competition."
In a lengthy statement on his Web site, Armstrong said the USADA had made "heinous" claims against him but that there was no point in fighting them in what he called an "unconstitutional" setting.
"I have been dealing with claims that I cheated and had an unfair advantage in winning my seven Tours since 1999," Armstrong said in his statement. "Over the past three years, I have been subjected to a two-year federal criminal investigation [which was closed without any action taken] followed by Travis Tygart's unconstitutional witch hunt. The toll this has taken on my family and my work for our foundation [for cancer research] and on me leads me to where I am today -- finished with this nonsense."
Armstrong had tried to have the USADA's actions dismissed by a federal judge but that bid was rejected.
"If I thought for one moment that by participating in USADA's process, I could confront these allegations in a fair setting and -- once and for all -- put these charges to rest, I would jump at the chance," Armstrong said. "But I refuse to participate in a process that is so one-sided and unfair.
"Regardless of what Travis Tygart says, there is zero physical evidence to support his outlandish and heinous claims. The only physical evidence here is the hundreds of controls I have passed with flying colors. I made myself available around the clock and around the world. In-competition. Out of competition. Blood. Urine. Whatever they asked for I provided. What is the point of all this testing if, in the end, USADA will not stand by it?"
The USADA made its charge against Armstrong two months ago. It said blood samples from 2009 and 2010 were "fully consistent with blood manipulation."
"I am a retired cyclist, yet USADA has lodged charges over 17 years old despite its own eight-year limitation," Armstrong said. "As respected organizations such as UCI have made clear, USADA lacks jurisdiction even to bring these charges.
"The international bodies governing cycling have ordered USADA to stop, have given notice that no one should participate in USADA's improper proceedings, and have made it clear the pronouncements by USADA that it has banned people for life or stripped them of their accomplishments are made without authority."