Nowadays, due to the ever-increasing number of electronic devices and communication systems that use high-frequency electromagnetic waves, a significant level of electromagnetic energy is available in the environment that is not entirely used. In this work, a complete electromagnetic harvesting system using a rectenna is proposed to collect this energy and feed a temperature measurement module. The rectenna is constituted by a combination of a microstrip antenna that captures the electromagnetic energy and a rectifier circuit that converts it into electric energy in direct current (DC) form to feed ultra-low-power loads. The proposed system uses a rectangular microstrip antenna, designed and optimized by using the Computer Simulation Technology (CST R ⃝) software to operate at 2.45 GHz. This designed antenna presents a measured reflection coefficient lower than −20 dB at the operating frequency with a maximum gain equal to 7.26 dB. In addition, a voltage doubler rectifier circuit is designed and optimized by using the Advanced Design System (ADS R ⃝) to match the impedance of the designed antenna to reduce the reflection losses between these two modules, achieving maximum measured efficiency of approximately 33%. Furthermore, a boost converter circuit is designed for the power management between collected and delivered powers to the sensor and to provide appropriate voltage levels to feed the temperature measurement module. This module consists of an ultra-low-power microcontroller and a temperature sensor that operates in the range of 1.8-3.6 V. The procedures for designing and testing each module of this system are detailed. Finally, a prototype is built and tested under different operating conditions to confirm its functionality and feasibility. These tests show that the proposed system can operate without batteries, only with the harvested electromagnetic energy dispersed in the environment, even from modulated and pulsating sources, as is the case of commercial routers.