Both maternal substance use and unpredictability of her 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 59 mother-child dyads, 34 with and 25 without maternal substance use. We also examined the association between unpredictability of maternal signals and children's cognitive development. Maternal sensory signals were evaluated with videorecorded 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 no differences in sensory signals between substance-using and non-using mothers. Higher unpredictability of maternal sensory signals was associated with poorer child cognitive and language development at 24 months and with poorer nonverbal reasoning at 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.