Victor Gevers, a security expert, said he was able to access the US President's direct messages, change his profile and post tweets in his name, after his fifth guess last Friday proved to be correct.
Gevers subsequently took screenshots which he shared with Dutch newspaper de Volkskrant.
He told the publication: "I expected to be blocked after four failed attempts. Or at least would be asked to provide additional information."
But with such huge power in his hands, Gevers said he did nothing more than email Trump - using an account of the President's he had previously acquired - advising him to enable two-step verification and suggesting he use a stronger password, such as '!IWillMakeAmericaGreatAgain2020!'.
Twitter has since rejected the alleged hack, stating there is no evidence to support Gevers' claim.
A spokesperson said in a statement: "We've seen no evidence to corroborate this claim, including from the article published in the Netherlands today.
"We proactively implemented account security measures for a designated group of high-profile, election-related Twitter accounts in the United States, including federal branches of government."
If Gevers isn't fibbing, then this will actually be the second time he has had access to Trump's account.
For years you had a President who apologized for America-now you have a President who is standing up for America, and standing up for YOU. So get your friends, get your family, get your neighbors and co-workers, and GET OUT AND VOTE. Together, we are going to #MAGA! pic.twitter.com/1EscDM2EuN
- Donald J. Trump (@realDonaldTrump) October 22, 2020
He and two others successfully managed to log in prior to the election in 2016, when the password was 'yourefired', according to Vrij Nederland.
Since his latest alleged hack, Gevers said he has noticed that two-step verification has been activated on Trump's account. He also said the American Secret Service got in touch this week to ask him a number of questions and also thank him for making them aware of the security leak.
However, the ease with which Gevers has been able to log in to arguably the most influential Twitter account on the planet on at least one occasion is cause for concern.
Gevers said: "Why is it possible for someone from a different time zone to log into such an important account? Why doesn't Twitter demand better passwords?
"If I can access his account, then foreign nations can do so as well, right? Why aren't the persons who are supposed to protect the President informed when someone reports that his account is unsafe?"