Former television presenter and writer Maxim Huerta had only just changed his Twitter account to reflect his new status as culture and sports minister.
On Wednesday he announced he was stepping down after news website El Confidencial reported Huerta was found to have evaded €218,322 ($256,462) in taxes when he was a television presenter 10 years ago.
Saying he had paid the fine, Huerta told a news conference in Madrid he was "absolutely innocent."
"I have paid the fine twice, at the time and now, here," he said, adding that the fine was a result of a change in the tax authority's criteria. He said that his tax payments were fully up to date.
A career television journalist, Huerta became a co-host of the morning television show El Programa de Ana Rosa in 2005 and stayed with the program until September 2015. But from 2006 to 2008 he was found to have used a company incorrectly in an attempt to reduce his tax bill. As a result, he was fined by the tax authorities.
Huerta said he was stepping down after a week in the job to prevent Prime Minister Pedro Sanchez's government from coming under attack.
"To defend what you love the most, sometimes you need to retire," Huerta wrote on Twitter.
Huerta did not say if he had told Sanchez about the tax fine before he was appointed.
Rooting out corruption
New socialist Prime Minister Pedro Sanchez had promised to root out corruption in public office after Prime Minister Mariano Rajoy lost a vote of a no-confidence in parliament after rulings in a corruption case involved his Popular Party (PP).
The long-running graft investigation known as the Gurtel case resulted in the former ruling PP treasurer Luis Barcenas being found guilty of evading more than €11.5 million in taxes between 2000 and 2009, and of taking €1.24 million in bribes in exchange for facilitating public contract awards.
The PP was also found guilty as a legal entity by the three-judge panel in Madrid of benefiting financially from the corrupt practices. It was ordered to pay €245,492.
As well as stepping down as premier, Rajoy also resigned as head of the PP. His successor has yet to be appointed.
jm/se (Reuters, AFP, AP)