An intensity-based optical fiber sensor using single-mode-multimode-single-mode fiber sections concatenation is proposed and investigated for use as an intrusion detector. The sensor is composed with simple FC/PC connections between the fiber sections. It is found to be sensitive to intrusion disturbances applied to the multimode fiber section. The transmission spectra of the device under different conditions are measured and its operation as intrusion detector is demonstrated using a single-mode laser source. More than 1 dB variation in the detected power level was observed at a wavelength of 1543 nm. The device proposed is a simple intensity-based sensor, which can be used as a stand-alone device to detect intrusion events over its length or possibly interrogated in a quasidistributed intrusion detection system based on optical time domain reflectometry.