The law will force companies to track their supply of tungsten, tin, tantalum and gold, which are often used in computers and smartphones.
It was approved after years of delays amid intense opposition by business lobbyists who won a late implementation date of January 2021.
The tracking of mineral sources by companies will now be compulsory, a requirement that also exists in the United States but that the administration of President Donald Trump is working to undo.
"The vicious circle has now been broken," said MEP Iuliu Winkler, who shepherded the bill through parliament, but whose own centre-right European People's Party had opposed binding rules on the issue.
With the law, "compulsory checks for the importers will ensure that minerals or metals sold on the European market are not used to illegally finance conflicts," Winkler added.
Lobbyists that rejected compulsory rules had pointed to tech giants like Apple and Hewlett Packard that previously made efforts to ensure the traceability of their supplies.
Activists however warned that the law, which was first introduced in March 2014, contained a string of last-minute loopholes including exemptions for smaller companies.
"While the EU has sent a strong signal to a small group of companies, it has ultimately trusted that many more will continue to regulate themselves," said Michael Gibb of campaign group Global Witness.
The EU bill is inspired by the Dodd-Frank Act, a vast 2010 US financial reform law under which US companies must inform regulators if they use metals from DR Congo or neighbouring countries.
But Trump last month ordered a review of Dodd-Frank as part of his wish to deregulate the US economy.
"Unfortunately, some of the vibes we are picking up from Washington on this matter are not encouraging," said EU Trade Commissioner Cecilia Malmstroem during the debate.
This "underlines all the more the importance of European leadership here," she added.
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