A pilot reported they temporarily lost control of the aircraft while touching down due to wind shear which pushed the aircraft off the runway into the grass.

Narrative

Ferry Maintenance Flight from ZZZ1 Direct ZZZ to maintenance facility. Flight was uneventful until short final approach when severe windshear was experienced. I flew the airplane down to low approach altitude and relayed that to the control Tower at ZZZ. I realized the wind was not just as depicted or 'Reported' by the AWOS/ASOS on the field or the Air Traffic Controller on staff was reporting the winds to me on the field as I requested two more low approaches or had balked landings/Go-Arounds and closed traffic to get a feel for the wind. However; there are multiple windsocks on the field in different locations for the same Runway. After a couple low approaches; I felt comfortable and was lined up centerline to initial touchdown on the Runway (Runway XX @ZZZ). I decided to use no flaps to ensure less drift and more efficient aerodynamic directional control to safely land the airplane. Everything looked good and felt good on the last approach to touchdown. Then there was a sudden burst of windshear that made the airplane nearly uncontrollable as I had one tire on the ground (initial X-wind touchdown) and that was the moment I felt the gust (windshear). In that instance I had to make a decision to attempt another 'go-around/balked landing' or land and do my best to safely control the airplane to a full stop. I decided not to risk a potential stall or loss of directional control etc.(due to the low airspeed as I had touched down on the Runway) I had to somewhat follow the path of the extreme windshear semi-controllably get the airplane in a safe position. In order to do this the wind pushed me off of the Runway on to the grass where I felt it safer to stop the airplane and maintain control to the taxiway (away from any buildings; structures or persons). Once I got the airplane to a full stop I called the Controller to request taxi to a facility on field; however he asked me to shut the airplane down on the taxiway that I stopped at after leaving the Runway and passing through a grass area that I saw ahead of me to a safe stop away from any person; airplane; or property. This was the first time in my years of flying that I encountered such severe windshear that I felt it safer to leave a Runway rather than attempt more power for control to stay centered. I controlled the airplane to a safe position away from persons or or property etc. and safely walked away (with praise from managers) and no persons injured.