This study investigated whether unattended speech is processed at a semantic level in dichotic listening using a semantic priming paradigm. A lexical decision task was administered in which target words were presented in the attended auditory channel, preceded by two prime words presented simultaneously in the attended and unattended channels, respectively. Both attended and unattended primes were either semantically related or unrelated to the attended targets. Attended prime-target pairs were presented in isolation, whereas unattended primes were presented in the context of a series of rapidly presented words. The fundamental frequency of the attended stimuli was increased by 40 Hz relative to the unattended stimuli, and the unattended stimuli were attenuated by 12 dB [+12 dB signal-to-noise ratio (SNR)] or presented at the same intensity level as the attended stimuli (0 dB SNR). The results revealed robust semantic priming of attended targets by attended primes at both the +12 and 0 dB SNRs. However, semantic priming by unattended primes emerged only at the 0 dB SNR. These findings suggest that the semantic processing of unattended speech in dichotic listening depends critically on the relative intensities of the attended and competing signals.