In a speech outlining the Government's plans on leaving the EU at Lancaster House in London, the Prime Minister said both Houses of Parliament will vote on the final deal.
Mrs May said the plans "cannot mean membership of the single market".
She said: "Being out of the EU but a member of the single market would mean complying with the EU's rules and regulations that implement those freedoms, without having a vote on what those rules and regulations are.
"It would mean accepting a role for the European Court of Justice that would see it still having direct legal authority in our country.
"It would to all intents and purposes mean not leaving the EU at all."
The PM confirmed that the Government will put the final deal between the UK and the EU "to a vote in both Houses of Parliament before it comes into force".
But she did not make it clear whether a vote against the agreement would result in the UK remaining within the EU, or leaving the block without any kind of deal.
Here are the key points from her speech:
:: The UK will not remain a member of the single market
:: Both Houses of Parliament will get a vote on the final deal on Brexit
:: The Government wants a deal on the Customs Union
:: There will be a transitional period that could require negotiation
:: Britain will no longer be under the jurisdiction of the European Court of Justice
Remaining within the single market would require the free movement of people and accepting the jurisdiction of the European Court of Justice.
Instead, Mrs May said she would seek "the greatest possible access to the single market on a reciprocal basis, through a comprehensive trade agreement".
That would mean the "days of Britain making vast contributions to the European Union every year will end", although there could be some specific European programmes that the UK might want to contribute in.
She said she wanted the UK to remain part of a customs agreement with the remaining 27 EU states, but said she had an "open mind" over whether this would be through associate membership of the Customs Union or through another arrangement.
And she said the UK would regain control of its borders, would no longer be under the jurisdiction of the European Court of Justice and would work to maintain a common travel area with the Republic of Ireland as well as strengthening the "precious union between the four nations of the United Kingdom".
But she sounded a warning to the EU against trying to "punish" the UK, claiming a punitive Brexit deal would be "an act of calamitous self-harm" and adding "no deal for Britain is better than a bad deal for Britain".
She also said if Britain was excluded from access to the single market, it would be free to change the basis of its economic model to pursue alternative arrangements.