Douglas Carswell defected from Prime Minister David Cameron's Conservatives but was re-elected in his old seat of Clacton, southeast England, last week with a majority of over 12,000.
Farage grinned broadly as Carswell was sworn in, promising to be "faithful to her majesty Queen Elizabeth, her heirs and successors, according to law, so help me God".
It was not immediately clear where he would sit in the House of Commons as he walked away immediately after the ceremony.
"I'm reinvigorated. Democracy works... I think change is coming -- people in the country feel let down by the two-party system," he said.
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In response to questions about the reaction he was expecting from former Conservative colleagues, he said: "I've got a lot of friends in the Conservative party -- I've been slightly taken aback by the number of friendly messages I've had."
The party once dismissed by the prime minister as "fruitcakes, loonies and closet racists" wants Britain to leave the European Union and to severely restrict immigration and has capitalised on increasing disenchantment with Westminster's main political parties.
Carswell's win came on the same day as UKIP narrowly lost out on a shock victory in a second seat, Heywood and Middleton in northwest England, traditionally a stronghold of the main opposition Labour party.