Michael Martin, editor of the True Faith fanzine, said the association would push him "close to breaking point", while local MP and season-ticket holder Ian Lavery has vowed to boycott games.
But a statement from Newcastle said Wonga had pledged to invest heavily in the club's academy and community projects during the course of the four-year deal.
And managing director Derek Llambias said: "We are building a club that can regularly compete for top honours at the highest level.
"As everyone knows, a strong commercial programme is vital to this goal and I am delighted to welcome Wonga into the fold as our lead commercial partner.
"Throughout our discussions Wonga's desire to help us invest in our young playing talent, the local community and new fan initiatives really impressed us and stood them apart from other candidates."
And Newcastle boss Alan Pardew told Sky Sports News: "I'm a romantic, so the stadium change last year hurt me and hurt our team. It's one of those business decision that you can't get your head around at times as a football person.
"I made a point of saying that the same sign should go up, not a new one, the same sign, 'St James' Park', and I'm absolutely overjoyed with that."
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