© 2020, The Japan Society of Naval Architects and Ocean Engineers (JASNAOE). An opposing sea was generated in an experimental wave basin. Here, an opposing sea is defined as a sea in which the wave spectrum has two peaks that are separated almost 180° in direction. Such an opposing sea can be observed, for example, when wind waves coexist in the opposite direction with swells. The opposing sea generation is based on an actual sea spectrum hindcast by a third-generation wave model. The experiment was carried out in an actual sea model basin (ASMB) at National Maritime Research Institute. The ASMB is fully surrounded by 382 flap-type wave makers with incident wave absorbing capacity. The estimation of the directional spectrum by the maximum likelihood method (MLM) revealed that the opposing sea was successfully generated in the basin, although an analysis of the corresponding numerical wave field indicated that the MLM inherently estimated the spectrum with wider directional spreading. The reproduction of the opposing sea in this study indicates that any deep-sea wave field with arbitrary directional spectrum can be reproduced in such a wave basin with absorbing wave makers on the periphery.