Maxim Huerta, who was named to the cabinet of Prime Minister Pedro Sanchez last week, said he had paid a fine related to his declared income in 2006 and 2008, and was "absolutely innocent".
"I have paid the fine twice, at the time and now, here," he told a news conference in Madrid, saying the penalty had resulted from a change in the tax authority's criteria.
Huerta added that he was fully up to date with his tax payments.
Sanchez toppled his predecessor, Mariano Rajoy, in a confidence vote over a long-running corruption scandal.
El Confidencial newspaper had reported that Huerta had been found to have avoided paying tax amounting to more than 200,000 euros ($235,700).
Spaniards have been enthusiastic so far in their reception of Sanchez's Socialist government, which broke with six years of right-wing domination and contains more women than men in ministerial posts for the first time in the country's history.
His party rebounded in opinion polls after taking power, overtaking rivals which had poached its voters in recent years, including market-friendly Ciudadanos and anti-austerity Podemos.
Despite these encouraging signs, Sanchez will have his work cut out to make any major policy changes, as he controls less than a quarter of the seats in parliament. ($1 = 0.8485 euros)
(Reporting by Isla Binnie; Editing by Gareth Jones)