Maternal substance use and unpredictable maternal sensory signals may affect child development, but no studies have examined them together. We explored the unpredictability, frequency and duration of maternal sensory signals in 52 Caucasian mother-child dyads, 27 with and 25 without maternal substance use. We also examined the association between unpredictable maternal signals and children's cognitive development. Maternal sensory signals were evaluated with video-recorded dyadic free-play interactions at child age of 24 months. Children's cognitive development was evaluated with Bayley-III at 24 months and with WPPSI-III at 48 months. We found similar unpredictability, frequency and duration of sensory signals between substance-using and non-using mothers. Higher unpredictability of maternal sensory signals was robustly linked with poorer child cognitive development at 24 months. The link persisted, although weakened to 48 months. Unpredictability of maternal sensory signals may be a vital parenting aspect shaping children's development, but more research is needed in high-risk groups.