The former deputy chairman of the Conservative Party jumped ship to the party founded with the support of Nigel Farage.
The move came just weeks after Mr Anderson sparked a furious backlash with an "Islamists" rant against Sadiq Khan which led to his suspension as a Tory MP.
Reform leader Richard Tice, who has previously been described by Mr Anderson as a "Poundshop Nigel Farage", welcomed the MP to his party.
Announcing the defection, Mr Tice hailed Mr Anderson as the "champion of the Red Wall", even though he had warned previously that backing his new party would let Labour into power.
The party leader said: "We are going to replace the Tories in the Red Wall."
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