The arrests have marked the first time authorities discover concrete evidence of the group's finance networks in Europe.
In Morocco, police have detained over 50 people for trying to establish a province for the group.
Spanish police on Wednesday arrested two Moroccan brothers, aged 33 and 22, in the northeastern city of Girona for helping finance the "Islamic State" ("IS") militant group's operations in Syria and Iraq, Spain's interior ministry said in a statement.
Police discovered a large amount of cash and telecommunications data at the brothers' home.
The information is expected to assist authorities in exposing the militant group's presence in Europe, police said.
"Investigators found that the two suspects, along with their brother who died in Syria, managed to send funds to financial administrators of the 'Islamic State' using false identities attributed to the terrorist organization's managers," the Interior Ministry said.
"These suspected identities form part of the 'Islamic State's' economic recruitment center at the international level, as evidenced by international contacts identified in the investigation," the ministry added.
Spanish authorities said the investigation marked the first time concrete evidence of European money transfers to the Islamic State, adding that the arrests will help neutralize one part of the group's financial support on the continent.
At least 30 people have been arrested in Spain this year over suspected links to Islamist militant groups, including IS, the ministry noted.
Following a series of attacks in Europe claimed by the Islamic State, Spain has raised its terrorism alert to its second-highest level.
In 2004, a local cell supportive of al-Qaeda set off bombs targeting trains in Madrid, killing nearly 200 people and injuring thousands more.
Meanwhile, Moroccan authorities arrested 52 suspected accused of planning to set up a branch of the Islamic State in the North African country.
The arrests foiled "terrorist plots at a very advanced stage of preparation on prisons, security establishments, festivals and leisure centers in several cities of Morocco, apart from assassinations of security officials, soldiers and tourists," the Moroccan interior ministry said.