The European Commission said the companies, which had looked to the deal as one way to tackle overcapacity and other challenges in the steel industry, had not done enough to allay its concerns.
"Steel is a crucial input for many things we use in our everyday life, such as canned food and cars. Millions of people in Europe work in these sectors and companies depend on competitive steel prices to sell on a global level," European Competition Commissioner Margrethe Vestager said in a statement.
The Commission said imports from third countries would not have been able to offset potential price hikes resulting from the deal.
This is Vestager's third veto this year, after she blocked Siemens' bid for Alstom, and German copper company Wieland-Werke AG to buy a business unit from Aurubis, Europe's biggest copper smelter, in February.
ThyssenKrupp and Tata Steel last month flagged the EU veto after they declined to offer further concessions to address regulators' concerns.
German steel-to-submarines group ThyssenKrupp will now list its elevator business, its most profitable unit worth an estimated 14 billion euros and twice the parent group's market value.
It may also consider new ownership structures for its car parts, plant engineering and marine systems units.
(Reporting by Foo Yun Chee; editing by Robin Emmott)
By Foo Yun Chee