Bamboo, due to its physical and mechanical properties, has significant potential for incorporation in civil construction. However, when subjected to specific loads or certain conditions, bamboo may become vulnerable, which is why technical standards recommend filling the bamboo with mortar or similar compounds for reinforcement. This study aimed to evaluate the mechanical behavior of bamboo when filled with cementitious composites reinforced with sisal fibers. For this purpose, bamboo specimens of the Bambusa vulgaris species were collected from the State University of Feira de Santana. After drying, the bamboo culms were cut into 20 cm lengths, both with and without nodes. The bamboo was filled with mortar and sisal fiber-reinforced composites, and four-point bending tests were performed on the described types of bamboo. The results showed that bamboo filled with the reinforced composite had a higher load capacity compared to unfilled bamboo and bamboo filled with mortar. The load versus deformation curves indicated that the post-cracking behavior of bamboo filled with the reinforced composite exhibited multiple peaks after the maximum load, due to the collaboration of the fibers in the mix, which was consistent with the failure type of this specimen. In contrast, bamboo filled with mortar showed fractures at the support points. It can be concluded that the incorporation of the fiber-reinforced composite significantly improves the bending behavior of bamboo.