The National Crime Agency has found "no evidence" of criminal offences after allegations against Leave.EU and its founder Arron Banks.
The agency launched an investigation into the pro-Brexit campaign group after it was fined £70,000 by the Electoral Commission in May last year.
But the NCA said it would not take any further action against Leave.EU, its chief executive Liz Bilney or Mr Banks.
Tweeting after the ruling, Mr Banks said: "Victory is sweet."
Earlier this month, a criminal investigation into Leave.EU was also dropped by the Metropolitan Police as there was "insufficient evidence" to justify any further inquiry.
In response to the NCA decision, the Electoral Commission said it stood by the need for the investigations to ensure "voters have transparency" over political funding.
In a May 2018 report, the commission said Leave.EU had exceeded the spending limit for "non-party registered campaigners" by at least 10% by failing to include at least £77,380 in its spending return.
It also referred Ms Bilney to the police, saying its investigation found she had committed four offences, including submitting an inaccurate spending return and exceeding the spending limit.
But she claimed the investigations had stemmed from Remain-backing MPs "desperately trying to overturn the result of the referendum".
Brexit Party leader and former Leave.EU campaigner Nigel Farage said he was "pleased" the NCA had dropped its investigation, but "heads must roll" at the Electoral Commission.
Leave-backing Labour MP Kate Hoey called for those who "condemned Leave.EU to apologise".
Leave.EU was a rival to Vote Leave, which was designated as the official Leave campaign during the 2016 referendum campaign.