The cardiac action potential (AP) is critical for initiating and coordinating myocyte contraction. In particular, the early repolarization period of the AP (phase 1) strongly influences the time course and magnitude of the whole‐cell intracellular Ca²⁺ transient by modulating trans‐sarcolemmal Ca²⁺ influx through L‐type Ca²⁺ channels (I_Ca,L) and Na‐Ca exchangers (I_Ca,NCX). The transient outward potassium current (I_to) has kinetic properties that make it especially effective in modulating the trajectory of phase 1 repolarization and thereby cardiac excitation‐contraction coupling (ECC). The magnitude of I_to varies greatly during cardiac development, between different regions of the heart, and is invariably reduced as a result of heart disease, leading to corresponding variations in ECC. In this article, we review evidence supporting a modulatory role of I_to in ECC through its influence on I_Ca,L, and possibly I_Ca,NCX. We also discuss differential effects of I_to on ECC between different species, between different regions of the heart and in heart disease.