A patient’s respiratory rate can be measured based on the thoracic excursion during inhalation and exhalation. For this it was implemented a wearable antenna–sensor made of cotton/polyester (polycot) fabric and flexible copper was designed in electromagnetic field simulation software CST Studio Suite to operate at an industrial, scientific, and medical frequency band (ISM) of 2.4 GHz. The measurement protocol used involved conducting a thoracic anamnesis in order to manually measure the expansion of the thoracic cage during the patient’s inhalation and exhalation, as a way to validate the measurements. A wearable patch antenna was used as the thoracic deformation sensor, which allowed for anatomical adjustment to the curvature of the patients’ chests. According to the findings, the variation in the resonant frequency of the antenna–sensor was linear and decreasing in the range of 2.15–2.6 GHz (bandwidth of 450 MHz), allowing for the detection of linear changes in thoracic excursion, with a sensitivity inversely proportional to the applied effort of −2.56 MHz/µ[Formula: see text]. These results were obtained by placing the sensor in the lateral region of the thorax for patients with obesity and respiratory pathologies. The satisfactory results demonstrate a novel instrumental technique for measuring thoracic deformation using a wearable patch antenna as a physiological sensor. This is the initial step toward the automation of the proposed measurement protocol in smart digital devices for respiratory rate monitoring in biomedical applications.