Soria has denied all wrongdoing, but said he was stepping down to limit any damage to the caretaker government, the People’s Party (PP) of Prime Minister Mariano Rajoy.
The Spanish media claimed to have documents proving that he headed an offshore firm with his brother in the British island of Jersey.
The resignation comes at a time when Spain is facing the likelihood of a second general election in June after an inconclusive vote in December.
The latest polls have shown the PP gaining ground, despite a string of corruption scandals involving regional politicians this year, as voter’s tire of left-wing parties’ failure to put aside differences to form a coalition government.
Leaks from the Panama-based law firm Mossack Fonseca have embarrassed several world leaders and shone a spotlight on the shadowy world of offshore companies by revealing the financial arrangements of prominent figures.
Icelandic Prime Minister Sigmundur David Gunnlaugsson stepped down earlier this month after documents linked him to an offshore company.