Traditional Japanese shoyu is made with soybeans only. Tamari is made with wheat. It incredibly unfortunate, then that Kikkomen, when it tried to distinguish its naturally brewed soy sauce from the hydrolysed soy protein on the market, decided to call its shoyu "tamari" in the US (and various other places). It's gotten so bad that "tamari" is often the one that doesn't have wheat in it abroad. However, at least for home cooking, if you buy a decent Japanese shoyu, it will definitely not have wheat.