The 33-year-old - a World Cup winner with France in 2018 - played with Inter Miami co-owner David Beckham at Paris St-Germain before joining Juve in 2017.
"I hope we will win a lot of trophies together," said Matuidi. "It's a big challenge for me."
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Inter Miami have lost all five matches so far in their debut MLS season, which is set to resume on 22 August.
"I couldn't be happier to welcome my friend Blaise to Inter Miami," said Beckham, the former England and Manchester United midfielder.
"He is an exciting and gifted player and a great person. To have a World Cup winner of Blaise's quality in our new team is such a proud moment - for us as owners and for our fans."
Matuidi, who will wear the number eight shirt, has been a league title winner in seven of the past eight seasons - four times in France with PSG and three times in Italy with Juventus - and started the 2018 World Cup final as France beat Croatia 4-2.