This paper presents the decentralized L1 adaptive controller for a class of large-scale systems with unknown timevarying interaction parameters. The design of decentralized L1 adaptive controller uses the states from decentralized identifiers. The L1 adaptive controller for each subsystem ensures uniformly bounded transient and asymptotic tracking for subsystem's input and output, simultaneously. The performance bounds between the closed-loop adaptive system and the closed-loop large-scale reference system can be systematically reduced by increasing the adaptation rate. Simulation results are provided to validate the effectiveness of the proposed control scheme.