Systems and methods for detecting atrial arrhythmias such as atrial tachyarrhythmia (AT) are described herein. An AT detection system may include a heart rate detector circuit configured to detect, from a physiological signal, representative ventricular heart rates within heart rate analysis windows. An atrial tachyarrhythmia detector circuit may perform an initial rate detection using a first ventricular heart rate statistic generated from the representative ventricular heart rates, and to perform a sustained arrhythmia detection using one or more second ventricular heart rate statistics generated within a second plurality of heart rate analysis windows during a specific duration. An AT event is detected if the second ventricular heart rate statistics satisfy a specific condition throughout the specific duration. The AT detection system may include an output unit that may output the detected AT to a user or a process.