We propose and experimentally demonstrate the use of polarization-nulling with off-center narrow-band filtering to realize a new polarization-mode dispersion (PMD)-insensitive in-band optical signal-to-noise ratio monitoring scheme. When applied to a 39.81-Gb/s 2.5-ps full-width at half-maximum return-to-zero on-off keyed optical time-division-multiplexing system with 10and 20-ps differential group delay, the monitoring errors were reduced from >20 dB to <0.9 and <1.6 dB, respectively, hence demonstrating the feasibility of this scheme even under severe PMD degradation. This technique is simple and can be easily upgraded for use in higher data-rate optical systems.