Piezoelectric materials have been broadly utilized in acoustic emission sensors, but are often hindered by the loss of piezoelectric properties at temperatures in the 500°C to 700°C range or higher. In this paper, a piezoelectric acoustic emission sensor was designed and fabricated using yttrium calcium oxyborate (YCOB) single crystals, followed by Hsu-Nielsen tests for high-temperature (>700°C) applications. The sensitivity of the YCOB sensor was found to have minimal degradation with increasing temperature up to 1000°C. During Hsu-Nielsen tests with a steel bar, this YCOB acoustic sensor showed the ability to detect zero-order symmetric and antisymmetric modes at 30 and 120 kHz, respectively, as well as distinguish a first-order antisymmetric mode at 240 kHz at elevated temperatures up to 1000°C. The frequency characteristics of the signal were verified using a finite-element model and wavelet transformation analysis.