A good diagnostic accuracy of smartphone-based electrocardiography (ECG) in the evaluation of heart rate, heart rhythm, and ECG measurements has been reported in humans, dogs, and cats. The aim of this study was to assess the feasibility of smartphone-based electrocardiography in horses. Fifty healthy adult horses were enrolled. Standard base-apex ECG and smartphone ECG were simultaneously recorded in each horse. All ECGs were reviewed by one blinded operator, who judged whether tracings were acceptable for interpretation and performed electrocardiographic measurements. Agreement between smartphone and standard base-apex ECG in the analysis of tracings was evaluated. Smartphone ECG tracings were interpretable in 48 of 50 (96%) cases. A perfect agreement between smartphone and standard ECG tracings was found in the assessment of heart rate. Heart rate automatically measured by the smartphone application was not reliable. In terms of electrocardiographic waves and interval duration, minimal differences of no clinical value were found between smartphone and standard ECG. Agreement was found for QRS complex polarity evaluation, but not for P wave polarity. Baseline artifacts were rare but significantly higher in the smartphone ECG tracings than those in standard ECGs. The smartphone ECG can record single-lead ECG tracings of an adequate quality for interpretation in horses. The smartphone ECG could represent an additional tool for the electrocardiographic evaluation of horses but is not a substitute for the standard base-apex ECG.