Abstract Electrochemical intercalation and storage of alkali metal ions into the graphite interlayer space is a key ingredient for commercial rechargeable ion batteries. While this has been exceptionally fruitful for lithium, the use of other more abundant chemical species such as potassium has fallen behind due to their less favorable ionic radius and host–guest interactions. Here, we deploy a device level on-chip cell architecture to study the reversible intercalation of potassium into the single van der Waals gallery of a graphene bilayer at room temperature and monitor in a time-dependent manner the electronic properties of the graphene bilayer during the insertion and extraction processes. The potassium diffusion is highly reversible and a charge transfer up to about 2.5 × 10¹³ cm⁻² is achieved. The diffusion coefficient well exceeds 10⁻⁷ cm² s⁻¹ at room temperature, an order of magnitude larger than previously reported values in graphite and other compounds.