Spoken word recognition is achieved via competition between activated lexical candidates that match the incoming speech input. The competition is modulated by prelexical cues that are important for segmenting the auditory speech stream into linguistic units. One such prelexical cue that listeners rely on in spoken word recognition is phonotactics. Phonotactics defines possible combinations of phonemes within syllables or words in a given language. The present study aimed at investigating both temporal and topographical aspects of the neuronal correlates of phonotactic processing by simultaneously applying ERPs and functional near-infrared spectroscopy (fNIRS). Pseudowords, either phonotactically legal or illegal with respect to the participants' native language, were acoustically presented to passively listening adult native German speakers. ERPs showed a larger N400 effect for phonotactically legal compared to illegal pseudowords, suggesting stronger lexical activation mechanisms in phonotactically legal material. fNIRS revealed a left hemispheric network including fronto-temporal regions with greater response to phonotactically legal pseudowords than to illegal pseudowords. This confirms earlier hypotheses on a left hemispheric dominance of phonotactic processing most likely due to the fact that phonotactics is related to phonological processing and represents a segmental feature of language comprehension. These segmental linguistic properties of a stimulus are predominantly processed in the left hemisphere. Thus, our study provides first insights into temporal and topographical characteristics of phonotactic processing mechanisms in a passive listening task. Differential brain responses between known and unknown phonotactic rules thus supply evidence for an implicit use of phonotactic cues to guide lexical activation mechanisms.