In computing, buffer underrun or buffer underflow is a state occurring when a buffer used for communicating between two devices or processes is fed with data at a lower speed than the data is being read from it. The term is distinct from buffer overflow, a condition where a portion of memory forms a buffer of a fixed size yet is filled with more than that amount of data. This requires the program or device reading from the buffer to pause its processing while the buffer refills. This can cause undesired and sometimes serious side effects because the data being buffered is generally not suited to stop-start access of this kind.

In terms of concurrent programming, a buffer underrun can be considered a form of resource starvation.

The terms buffer underrun and buffer underflow are also used for meaning buffer underwrite, a condition similar to buffer overflow, but where the program is tricked into writing before the beginning of the buffer, overriding potential data there, like permission bits.