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One of the challenges of using an endoscopic robot for natural orifice transluminal endoscopic surgery (NOTES) is the ability to adjust its joint stiffness. The endoscopic robot joint needs low stiffness to move through paths in the human body without damaging tissues and high stiffness to keep its shape. This paper presents a variable stiffness manipulator for the endoscopic robot in NOTES. The manipulator has a backbone tube that uses a thermoplastic material, polyethylene terephthalate glycol (PETG), with a temperature effect to change the stiffness. The backbone of the robot is designed with a heating coil, cooling tube, and thermal sensor to adjust the temperature. Analysis and experiments were conducted to evaluate and find the backbone structure with a large range of stiffness modulations, a short heating time, and a short cooling time. This paper also presents a temperature control system that controls the temperature to maintain the stiffness of the robot in real-time. The stiffness characterization, heating time, and cooling time of the robot and the response of the temperature controller, are tested experimentally. The results confirm that the endoscopic robot can be changed and maintained at some stiffness values.