An anticausal system or acausal system is a system that depends on both the past and the future, as opposed to a causal system which only depend on the past. This is a term often used in control theory and digital signal processing (DSP).

An example of acausal signal processing is a signal produced from another signal by looking both forward and backward in time. This type of processing cannot be done in realtime, but must be done after a signal is captured by post-processing methods.