Boron nanosheets with high surface hydrophilicity and proton conductivity are promising for ionic devices. However, their poor electrical conductivity has severely limited their applications in resistive-type electronic devices. Herein, electrically conductive three-dimensional boron/MXene hollow spheres were prepared by a freeze-drying-assisted self-assembly method. The composites were fabricated into resistive-type humidity sensors, which showed high sensitivity with a three-orders-of-magnitude variation in resistance, a wide detection range (11−97%), fast response and recovery time (4 s/3.8 s), and long-term stability. The excellent humidity sensing performance can be attributed to the abundant hydrophilic groups on both the inner and outer surfaces of open spheres as well as to a coupled hole/proton conducting process. As a proof of concept, real-time remote monitoring of human respiration was achieved by integrating our humidity sensor with a Bluetooth module, enabling differentiation of breathing frequency as rapid as 0.8 s.