Learning to recognize morphemic boundaries is essential for fluent language use. In languages with rich morphology, the question of morphological learning is particularly relevant, because the abundance of bound morphemes represents one of the most challenging aspects of learning. However, neural mechanisms underlying the acquisition of novel morphological units remain largely unexplored. Here, we investigated the online build-up of novel suffix representations in the left frontal and temporal cortices. We trained 19 native Finnish-speaking participants with novel suffixes through a word-picture association task. Following this short semantic training session, we used magnetoencephalography (MEG) to record the participants’ brain responses to novel suffixes combined with real and pseudoword stems. During the MEG recording, the affixed stimuli were presented in a passive listening task without attention focused on the presented sounds. Half of the novel suffixes were trained and half untrained, and real Finnish suffixes served as acoustic controls. We compared neural sources measured in the early and late phases of short passive exposure to the novel suffixes. We found an overall response enhancement for suffixes with real stems, in contrast with a general response decrease for pseudoword stems, implying that suffixes were morphologically decomposed when they were attached to a recognizable stem. Moreover, both types of novel suffixes elicited lower fronto-temporal source activations than real suffixes, suggesting that such a brief exposure may not be sufficient for the integration of novel morphological units into the mental lexicon. However, semantic training appeared to modulate the learning process already in the early stages, indicated by the different response patterns evoked by trained and untrained novel suffixes. Responses for untrained suffixes combined with real stems, in turn, increased over the course of the passive listening task, possibly reflecting memory trace formation. Previously trained suffixes, instead, showed a consistent activation pattern throughout the task. The different activation patterns for the two suffix types may reflect distinct underlying neural processing mechanisms. Untrained suffixes with merely phonological information may be processed through procedural memory, while declarative memory could be involved in case more explicit, semantic information is attached to the novel morphemes.