The decision by antitrust regulators was subject to Dupont selling "major parts" of its global pesticide business, said the European Commission, the EU's executive arm.
"Due to significant commitments on products and the worldwide research and development organisation, the merger of Dow and Dupont can be approved," EU Competition Margrethe Vestager said.
Dow Chemical and DuPont, two of the oldest US companies, announced their tie-up in December 2015 to create the world's biggest chemical and materials group.
The companies are dominant players for a huge range of chemical products, but the manufacturing of pesticides and fertilisers has drawn the most attention.
The products developed by the Dow and DuPont "affect each and everyone of us. They literally affect our daily bread," Vestager told a news conference in Brussels.
The Dow Dupont merger is part of a broader wave of consolidation in the agro-chemicals sector with the EU expected to decide on the $43 billion takeover bid by ChemChina for Swiss rival Syngenta in the coming days.
To win approval, the companies agreed to sell off key DuPont pesticide units, including sensitive research and development capabilities.
Opponents to the deals warned that the tie-ups would deepen threats to the environment, punish farmers worldwide with higher prices and bring a boost to the controversial genetically-modified crops industry.
In the coming months, the EU is expected to also decide on German giant Bayer's $66 billion offer for US firm Monsanto, another mega merger in the industry that has angered activists.
The green light for the Dow-Dupont deal spares the EU from angering the United States.
Vestager, who visits Washington on Friday, has not been shy of taking on major US companies, pushing through a series of anti-competition probes of such icons as Apple, Google and Amazon.
Some US critics say she unfairly targets American companies but Vestager insists she had simply applied European Union competition rules.