Ciara‘s son with rapper Future is legally a Wilson … ’cause TMZ has learned the singer locked down a name change for her eldest, swapping out his dad’s last name for her husband Russell Wilson‘s.

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.” That sparked 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. We’re told Russell didn’t adopt Ciara’s eldest, but the name change is legit.