During an interview with Jeremy Paxman, the Prime Minister vowed to be a "difficult woman" to seal the best possible deal.
She said: "I'm not prepared to sign up to a bad deal to the UK. We're in there to get the best deal for the United Kingdom.
"I'm optimistic because I think the European Union will want to continue their relationship with us. We'll be negotating hard."
Mrs May was repeatedly asked if she had changed her mind on Brexit after supporting Remain during last year's referendum.
She said: "I set out my reasons for deciding, on balance, we should stay within the EU. I voted to remain, I campaigned to Remain.
"The British people were given the choice and the British people decided that they wanted the UK to leave the EU."
Jeremy Paxman grilled PM Theresa May on her Brexit stance Theresa May's most powerful quotes Tue, March 28, 2017 Getty Images 1 of 8
'I will be ruthless in cutting out waste, streamlining structures and improving efficiency'
That's why I thought it was right to call an election
The PM also defended her decision to call a snap general election, saying she needs a large mandate to make Brexit a success.
She said: "What became clear and is increasingly clear in this election, is the desire of other parties to frustrate the will of the British people.
"That's why I thought it was right to call an election."
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She defended her decision to call a snap general election
Mrs May insisted net migration would fall once the UK regains control of its borders after Brexit.
She said: "It's a constant work to ensure you're dealing with immigration and ensuring there is no more abuse in the system. There's more work to be done.
"We will of course have another tool to do this when we leave the EU as we'll be able to introduce rules for people coming within the EU."