Systems and methods for detecting atrial tachyarrhythmia are discussed. An exemplary atrial tachyarrhythmia detection system includes an arrhythmia detector circuit configured to receive physiologic information of a patient, generate a morphological similarity metric between the received physiologic information and a sinus rhythm (SR) template representing a morphology of conducted sinus beats during normal SR, and generate a morphological variability metric indicative of a variability in morphology between heart beats in the received physiologic information. The arrhythmia detector circuit may detect an atrial tachyarrhythmia episode the morphological similarity and morphological variability metrics.