Focal left atrial tachycardias (AT) originating from the mitral annulus-aorta (MA-Ao) junction are rare and their mechanisms are unclear. We report a 35-year-old male with successful ablation of an exercise-induced focal AT due to triggered activity originating from the MA-Ao junction. The AT occurred spontaneously during treadmill exercise testing and was easily induced by an atrial extrastimulus and atrial burst pacing after intravenous administration of isoproterenol. The AT was terminated by an atrial extrastimulus as well as a bolus of 5 mg of adenosine 5'-triphosphate. The coupling intervals of the extrastimuli that induced the AT were positively correlated with the interval between the extrastimuli and the first beat of the AT, suggesting the triggered activity as a tachycardia mechanism. The AT was successfully eliminated by a focal ablation at the MA-Ao junction with the earliest atrial activation where fractionated atrial potentials were recorded. The MA-Ao junction should be recognized as an important arrhythmogenic region.