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This note focuses on the leader–follower formation control problem of underactuated surface vehicles (USVs) with the unknown environmental disturbance. Especially, due to the ocean environmental disturbance, the vehicle's desired-heading angle is altered violently. To address the constraint, a reduced-order extended state observer (ESO) is introduced. The sideslip angle of the vehicle is estimated online to amend the reference heading angle by virtue of the proposed observer. Furthermore, a novel adaptive formation control algorithm is presented to stabilize the dynamic error system. For the merits of the radial basis function neural networks (RBF NNs) and the minimal learning parameter (MLP) techniques, only two adaptive parameters require to be updated online to compensate for the perturbation from the model uncertainty and the environmental disturbance. Based on the proposed controller, it is illustrated that the desired formation pattern can be maintained without the effect of the sideslip angle. Meanwhile, all signals in the closed-loop system are proved to be with the semi-global uniformly ultimately bounded (SGUUB) stability. Finally, numerical simulations are conducted to demonstrate the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]