That figure represents a strong haul that will help fuel Cruz's operation at a critical juncture.
His cash flow over the last three months brings his total fundraising so far to $26 million. Cruz raised $10 million in the previous quarter, and $4 million in his first quarter as a candidate.
Cruz outpaces his rival Marco Rubio, who told donors earlier on Thursday that his campaign raised $6 million in the third quarter.
The Texas senator's success at raising money has been one of the surprising fundraising stories of the Republican race. His four affiliated super PACs raised $38 million as of the end of June thanks to eight-digit donations from three families to the groups, which can accept unlimited contributions.
The Cruz campaign did not respond to requests for comment Thursday evening.
Cruz worked the fundraising circuit hard during the traditionally slow summer months, winning donations in his hometown of Houston but also traveling to luncheons and receptions in places like Wyoming and Florida to raise cash.
Cruz's financial success has been a large part of his team's argument for why he will win the Republican nomination: Cruz fashions himself as the only well-funded conservative candidate, and he boasts of his financial success in nearly every stump speech. And he has set his sights on a national campaign that he says will not focus solely on Iowa, though the candidate is increasingly gunning for a top performance there.
But there are signs that other conservative alternatives are gaining steam: Ben Carson, powered by a legion of small-dollar donors, says his campaign raised $20 million in the third quarter. And Donald Trump has pledged to self-finance his campaign with his real estate fortunes.
But Cruz's money shop has nevertheless beat back candidates who seemed to have similar ideological appeal: Rand Paul has only raised $2.5 million in the third quarter, his campaign said. And the Texas senator has successfully won invitations to events hosted by many of the premier Republican financiers, including the summit hosted by Charles and David Koch and next weekend's conference put on by the Club for Growth.
Cruz allies and aides have expressed surprise throughout the summer
that their rivals have not been more competitive; many in Cruz's world have said they did not expect to be in second place financially at the end of the second quarter. And the summer months gave Cruz two chances to wage high-profile fights that may have motivated even more low-dollar giving: his unsuccessful attempt to defund Planned Parenthood and his crusade to thwart the White House's deal with Iran.
Cruz said in a radio interview last week that his campaign had received more than 300,000 donations, indicating he had received about 125,000 contributions during the third quarter.
"That's the strength of we the people rising up, and I got to tell you, it scares the living daylight out of the Washington cartel," he told conservative host Mark Levin.
The $12 million third quarter haul comes as Cruz struggles to set himself apart from a large group of mid-tier GOP candidates vying for the White House. Cruz was only at 5% in a recent NBC News/Wall Street Journal
poll, behind John Kasich at 6% and Jeb Bush at 7%.
Cruz came to the Senate in 2013 brandishing an anti-Washington message, and quickly made a name for himself by crusading against President Barack Obama's health care law and butting heads with party leaders. The government shutdown he helped orchestrate that fall angered Wall Street and many of its donors.