Background Supraventricular tachycardias (SVTs), despite having various anatomical substrates and pathophysiological mechanisms, frequently show similar electrocardiographic presentations. Objectives To locate and characterize atrial deflections (ADs) on 12‐lead electrocardiograms in dogs with sustained rapid SVT and assess the utility of different electrocardiographic variables in differentiating types of tachycardia. Animals Ninety‐two dogs with orthodromic atrioventricular reciprocating tachycardia, 17 with atrial flutter, 33 with focal atrial tachycardia recorded and confirmed by electrophysiological study, and 40 dogs with sinus tachycardia. Methods Atrial deflection position on the 12‐lead surface electrocardiogram was assessed according to the sequence of intracardiac activation. Its features were evaluated together with the relationship between AD and QRS complex interval (AD‐R) and QRS complex and AD interval (R‐AD). Results Orthodromic atrioventricular reciprocating tachycardia was characterized by an AD‐AD interval of 213 ± 30 ms, mean electrical axis (MEA) of AD of −90 (−90/−78)°, R‐AD interval of 75 (65‐80) ms, and R‐AD/AD‐R of 0.54 (0.45‐0.64). Atrial flutter was characterized by an AD‐AD interval of 199 ± 57 ms, MEA of 76° (72/81), R‐AD of 120 (72‐144) ms, and R‐AD/AD‐R of 0.81 (0.63‐1.13). Focal atrial tachycardia was characterized by an AD‐AD interval of 270 ± 38 ms, MEA of 49 (−72/76)°, R‐AD of 160 (120‐200) ms, and R‐AD/AD‐R of 1.45 (0.92‐1.67). Sinus tachycardia was characterized by an AD‐AD interval of 292 ± 31 ms, MEA of 66° (52/73), R‐AD of 215 (192‐222) ms, and R‐AD/AD‐R of 2.68 (2.25‐3.08). Conclusions and Clinical Importance Analyzing AD on 12‐lead electrocardiogram is helpful in differentiating the most common SVTs in dogs.