Previous studies have induced robust adult learning of non-native natural speech stimuli using explicit instructions and feedback. Here, Greek adults were exposed to non-native Hindi phonemes for six sessions in which they were unaware of the phoneme distinctions and the phonemes had no relevance to their main task. Stimuli were natural recordings of CV syllables (a retroflex or dental unvoiced stop followed by [a]) from a Hindi speaker. On each trial, participants heard two identical dental sounds and two retroflex sounds differing in intensity by an adaptively varied amount between 0.5–3 dB. The explicit task was to identify which pair differed in intensity, while the implicit learning goal was the phonetic contrast. After training, participants were tested on explicit phonetic identification and discrimination of trained and untrained stimuli spoken by the same and a novel speaker. The trained group outperformed an untrained group who were administered the same tests. Our results are compatible with a perceptual learning model where task-irrelevant stimuli (Hindi sounds) are learned due to a systematic pairing with task targets (intensity difference). Learning did not extend to the novel speaker, in agreement with previous studies showing that variability in training is crucial for generalization.