A bicycle rear sprocket adapter comprises a plurality of external spline teeth and a plurality of internal spline teeth. The plurality of external spline teeth is configured to engage with a bicycle rear sprocket assembly. The plurality of internal spline teeth is configured to engage with a bicycle hub assembly. At least two internal spline teeth of the plurality of internal spline teeth are circumferentially arranged at a first internal pitch angle with respect to a rotational center axis of the bicycle hub assembly. The first internal pitch angle ranges from 10 degrees to 20 degrees.