Lateral access control in downhole window joint is provided via a window joint. The window joint includes an interior cavity with a first end and a second end opposite to the first end, a first opening defined at the first end, and a lateral opening through a wall of the window joint and into the interior cavity between the first end and the second end. The window joint also includes a deflector where, in a first state, the deflector permits access between the first end and the second end and, in a second state, the deflector blocks access between the first end and the second end and permits access between the first end and the lateral opening by deflecting an object travelling through the lateral opening. The window joint also includes an access control system configured to transition the deflector between the first state and the second state.