The theory of grain boundary (the interface between crystallites, GB) structure has a long history1 and the concept of GBs undergoing phase transformations was proposed 50 years ago2,3. The underlying assumption was that multiple stable and metastable states exist for different GB orientations4-6. The terminology 'complexion' was recently proposed to distinguish between interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexion and transitions between complexions have been characterized, mostly in binary or multicomponent systems8-19. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems20-24. However, the direct experimental observation and transformation kinetics of GBs in an elemental metal have remained elusive. Here we demonstrate atomic-scale GB phase coexistence and transformations at symmetric and asymmetric [Formula: see text] tilt GBs in elemental copper. Atomic-resolution imaging reveals the coexistence of two different structures at Σ19b GBs (where Σ19 is the density of coincident sites and b is a GB variant), in agreement with evolutionary GB structure search and clustering analysis21,25,26. We also use finite-temperature molecular dynamics simulations to explore the coexistence and transformation kinetics of these GB phases. Our results demonstrate how GB phases can be kinetically trapped, enabling atomic-scale room-temperature observations. Our work paves the way for atomic-scale in situ studies of metallic GB phase transformations, which were previously detected only indirectly9,15,27-29, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement.