The former ArcelorMittal plant was liquidated in 2016 and has since been acquired by Pinnacle Steel & Vanadium Corporation, an American firm which hopes to turn the mill into a vanadium production hub.
China currently dominates production of the steel-strengthening element, which is used in the manufacture of some military components and magnets.
US Deputy Secretary of State Christopher Landau took aim at environmental regulations during a speech at the ceremony.
"Everybody wants a clean and safe environment," he said, "but at some point, layers of regulation upon regulation don't really materially help the environment."
Environmental standards "create a nightmare for people who are trying to run a business," he added.
Landau said he believed the US had lost out to its Chinese competitor after companies relocated to the Asian giant.
"We're dependent on a foreign country that may not have our best interests at heart, and so it is a strategic imperative for us as the United States to revitalize these kinds of projects," he said.
The plant's reopening will create 644 jobs on the island, according to Works and Infrastructure Minister Jearlean John.
"There is nothing more beautiful, frankly, than being able to see a plant like this that's been shuttered come back to life," Landau said.
Trinidad and Tobago is located just around 10 kilometers (six miles) off the Venezuelan coast, and US President Donald Trump counts Prime Minister Kamla Persad-Bissessar as a close ally.
The leader last year welcomed an American warship to the island as part of the US military deployment that eventually deposed Venezuelan president Nicolas Maduro.
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