A method and apparatus to encode/decode an audio signal, in which a bit rate for each bit plane can be controlled. A method of encoding an audio signal for each of a plurality of bit plane can include dividing the audio signal into a plurality of frequency bands and encoding the bit planes of the frequency bands from a low frequency band to a high frequency band, wherein, in the encoding the bit planes of the frequency bands, the bit planes are encoded from the most significant bit (MSB) to the least significant bit (LSB) within bits allocated for the frequency bands, and when there are allocated bits remaining after the encoding of the currently encoded frequency band, un-encoded bit planes corresponding to the MSB in a frequency band that has the fewest encoded bit planes among frequency bands with a lower frequency than the currently encoded frequency band are encoded using the remaining allocated bits. Accordingly, when encoding/decoding an audio signal, an encoding sequence of bit planes is determined so that an audio signal that significantly affects audio quality during decoding is first encoded, thereby reducing audio quality deterioration at a low bit rate.