Novel solid forms of tacedinaline (4-(acetylamino)-N-(2-aminophenyl)benzamide), including crystalline tacedinaline Forms A, B, and D, a novel crystalline tacedinaline TFA salt, and amorphous tacedinaline, are disclosed. Pharmaceutical compositions comprising crystalline tacedinaline Forms A, B, and D, the novel crystalline tacedinaline TFA salt, and/or amorphous tacedinaline, and methods of treating various conditions by administering those novel solid forms, are also disclosed.