The quantum anomalous Hall effect (QAHE) is a topological state of matter with a quantized Hall resistance. It has been observed in some two-dimensional insulating materials such as magnetic topological insulator films and twisted bilayer graphene. These materials are insulating in the bulk but possess chiral edge states carrying the edge current around the system. Here we discover a metallic QAHE in a topological insulator film with a magnetic sandwich heterostructure, in which the Hall conductance is quantized to $e^2/h$ but the longitudinal conductance remains finite. This effect is attributed to the existence of a pair of massless Dirac cones of surface fermions, with each contributing half of the Hall conductance due to the quantum anomaly. It is not characterized by a Chern number and is not associated with any chiral edge states. Our study offers insights into topological transport phenomena and topological metallic states of matter.

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• Quantum anomalous Hall effect
• 2-dimensional systems
• Topological insulators