Since the control performance of a proportional-integral-derivative (PID) control systems strongly depends on PID gains, lots of schemes for tuning PID gains have been proposed up to now. Recently, data-driven PID tuning schemes whose PID gains are directly determined from the closed-loop operating data attract attention. Lots of controlled objects are multi-input/multi-output systems, though almost data-driven PID tuning schemes target single-input/single-output systems. In this paper, a new design method of a multiloop self-tuning PID controller is proposed. The proposed scheme first employs a postdecoupler to remove the influence of mutual interference. The multiloop self-tuning PID controller is designed for the approximately decoupled system. The effectiveness of the control scheme is evaluated by numerical and experimental examples.