A bicycle operating device mounting assembly is basically provided with a handlebar mounting clamp and an adapter member. The handlebar mounting clamp has a clamp opening that defines a first diameter. The adapter member includes a connecting portion and a projecting portion. The connecting portion is detachably disposed within the clamp opening. The projecting portion projects from the clamp opening. The connecting portion has a second diameter that is smaller than the first diameter. The connecting portion is configured to clamp a handlebar as the handlebar mounting clamp is tightened in a state that the connecting portion is disposed within the clamp opening.