The couple welcomed a daughter named Avri on Tuesday in Los Angeles, according to a tweet from Downey.
The producing partners are already parents to son Exton, 2. Downey, 49, also has a 21-year-old son, Indio, from his previous marriage to Deborah Falconer.
The Downeys, who met on the set of the 2003 film Gothika, married in 2005.
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