In this letter, we propose an optimal subarray design method for wideband irregular subarrayed array with sidelobe cancellation (SLC). First, we present the space–time adaptive SLC structure for wideband irregular subarrayed array. The main channel adopts the wideband beamforming method with module-level time delay and subarray-level phase-shifting, and the auxiliary channels are connected with multitap transverse filter banks to provide temporal degree of freedom. Considering the quasi-exact partition of the main array and the position constraints of the auxiliary elements, then we establish the optimal subarray design method to achieve the best interference cancellation performance, and use a two-step optimization method to get the results. The obtained subarray structure has better interference cancellation performance than the traditional method. The effectiveness and potential of the proposed method are verified by different numerical experiments.