This study investigates the role of orthography in German vowel production by Polish native speakers (L1) with German as a second language (L2). Eighteen intermediate to advanced Polish L2 German learners and 20 German native speakers were recorded during a picture-naming task in which half of the experimental items were explicitly marked in their orthographic representation for their vowel length (short or long). Duration measurements revealed that explicit orthographic marking helped the Polish L2 German learners produce the short-long contrast more native-like. Regarding vowel quality, group differences were interpreted to have been caused by (in)congruencies between L1 and L2 grapheme-to-phoneme correspondences. These findings have important implications for models of L2 speech learning and pronunciation training.