At a height of 108 metres, it is taller than New York's Statue of Liberty -- while its weight of 36,000 tons is three times heavier than the Eiffel Tower in Paris.
The 2.1-billion-euro (USD 2.2-billion) structure sponsored by the European Bank for Reconstruction and Development (EBRD) has been edged into place over an existing crumbling dome that the Soviets built in haste when disaster struck three decades ago.
"We welcome this milestone in the process of the transformation of Chernobyl as a symbol of what we can achieve jointly with strong, determined and long-term commitment," EBRD president Suma Chakrabarti said in a statement.
Work on the previous dome began after a 10-day fire caused by the explosion was contained but radiation still spewed out of the stricken reactor.
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"It was done through the super-human efforts of thousands of ordinary people," the Chernobyl museum's deputy chief Anna Korolevska told AFP.
"What kind of protective gear could they have possibly had? They worked in regular construction clothes."
About 30 of the cleanup workers known as liquidators were killed on site or died from overwhelming radiation poisoning in the following weeks.
The United Nations estimated in 2005 that around 4,000 people had either been killed or were left dying from cancer and other related diseases.
But the Greenpeace environmental protection group believes the figure may be closer to 100,000.
Authorities maintain a 30-kilometre-wide (19-mile) exclusion zone around the plant in which only a few dozen elderly people live.