This protocol details our minimally invasive approach for implanting human thymus tissue under the kidney capsule of NSG mice. In contrast to our open abdominal approach, this approach is from the dorsal aspect of the mouse and requires only two interrupted sutures and 1-2 staples to close the incision. The technique can be applied to other strains of mice, though we have found the NSG kidney capsule to be more delicate, and thus more challenging to manipulate during surgery.