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.