How do children with developmental dyslexia process unattested or ill-formed phonological sequences in their native language? This question warrants attention because these children are primarily characterized by a phonological deficit. In this study, we support the hypothesis that intact phonological grammar allows segmenting and recognizing (pseudo)words through sensitivity to sonority markedness constraints. We administered a lexical decision task in silent reading to 21 French children with developmental dyslexia, comparing them with 21 chronological age-matched and 21 reading level-matched peers. Children were presented with words and pseudowords that either respected or transgressed syllable boundaries (⟨ar*gent⟩, money vs. ⟨a*rgent⟩ vs. ⟨arg*ent⟩). For pseudowords, we manipulated the sonority profiles of unattested intervocalic ⟨C1C2⟩ clusters from unmarked, well-formed (⟨rj⟩ in ⟨yrjyde⟩; high-fall) to marked, ill-formed clusters (⟨vl⟩ in ⟨uvlyde⟩; high-rise). Results confirmed preferences for syllable segmentation in words (⟨ar*gent⟩ is preferred to ⟨a*rgent⟩ or ⟨arg*ent⟩) regardless of distributional properties. We found a sonority projection effect that illustrated a gradient-based preference for sonority markedness constraints with pseudowords. However, pseudowords conforming to expected sonority-based segmentation (⟨yr*jyde⟩ or ⟨u*vlyde⟩) were more difficult to reject, possibly due to interferences from lexical attestedness. We discuss a phonological deficit that does not stem from degraded language-specific or universal phonological representations.