We explored the role of lexical stress in word segmentation as speech unfolds in time. We tested participants online with a Mouse Tracking listening experiment using temporarily ambiguous phrase pairs of the form "PAlo marron” vs. "paLOma roja”. These pairs were segmentally ambiguous in the first three syllables but differed in the location of lexical stress. Thus, use of stress cues would allow participants to disambiguate the phrases more quickly. We also manipulated the presence of lexical stress correlates in two conditions (stress natural and stress neutral) and found that lexical stress has an early impact in Spanish word segmentation that can affect how quickly and efficiently the speech signal is processed.