Raducanu, winner at Flushing Meadows in 2021, also required a wildcard to get into next week's event in Indian Wells after an injury plagued start to the season.
The British number one has not played since injuring her ankle at the Australian Open and was forced out of the ATX Open in Austin after contracting tonsillitis.
Thiem, the 2020 US Open champion who was once ranked as high world number three, has also had his injury struggles this year with the Austrian collecting just one singles win this season.
"Both these players have shown what it takes to win the biggest tournaments in the world, and we look forward to welcoming them to Miami to compete for the title here in a few short weeks," said tournament director James Blake.
The Miami Open runs from March 19 to April 2.
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