Landiolol, an ultra-short-acting β1-selective blocker, is more effective in controlling heart rate compared with digoxin in patients with atrial tachyarrhythmias and left ventricular dysfunction. However, the effect of atrial tachyarrhythmia type on the effectiveness of landiolol remains unknown. Hence, this study aimed to evaluate the effectiveness of landiolol in patients with atrial fibrillation (AF) and atrial tachycardia (AT), not limited to those with heart failure with a reduced ejection fraction. To this end, we evaluated the efficacy and safety of landiolol in managing tachycardiac AF and tachycardiac atrial flutter/AT in 44 patients with reduced left ventricular function. We found that while landiolol was effective in managing patients with AF and heart failure with a preserved or mid-range ejection fraction, however, it might be more challenging to control heart rate in patients with AT using a similar dosage of landiolol.