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Ciara 's son with rapper Future is legally a Wilson ... 'cause TMZ has learned the singer added Russell Wilson 's last to her eldest son's legal name.

ICYMI ... Ciara and Russ hit the US Open this week with their kids -- but when Russ posted a shot of Ciara and their family from the stands with the caption, "Mrs. @ciara Wilson Future Wilson & Sienna Wilson," fans zeroed in on the fact Future Zahir 's last name was written as "Wilson" ... sparking speculation Russell had adopted him.

Sources with direct knowledge tell TMZ ... Ciara has full custody of her 11-year-old, and "Wilson" was legally added to his name a few years back. He still has his dad Future's last name Wilburn in his full legal name. Play video content

Ciara and Russ -- married 9 years, parents to 3

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