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.