["Family Process, Volume 65, Issue 3, September 2026. ", "\nABSTRACT\nAlthough the implications of parental smartphone use for parenting have been discussed among researchers and practitioners, prior studies have mostly focused on the between‐person associations of problematic smartphone use with negative parenting behaviors. Therefore, using 5 days of daily diary data, the present study examined whether day‐to‐day fluctuation in maternal problematic smartphone use was associated with day‐to‐day fluctuation in maternal hostility as well as warmth toward children, on the same day and on the following day. Data were collected from mothers of 74 fourth graders (mean age = 9.20 years; age range = 8–10 years; 69% were girls) studying in five elementary schools in Hong Kong, China. On each of five consecutive days, mothers filled in an online questionnaire in the evening on their own smartphone use and their parenting behaviors during the day. Random intercept multilevel linear models within the Bayesian framework revealed that on days when mothers used smartphones more problematically (compared to their cross‐day averages), they showed more hostility and less warmth toward their children (compared to their cross‐day averages) on the same day, even controlling for fluctuation in parenting on the previous day. However, no evidence of lagged‐day associations was found. Theoretically, findings pointed to the potential role of parents' problematic smartphone use in understanding daily variation in both negative and positive parenting behaviors. Practically, findings underscored the possible utility of using community education and intervention programs to help parents monitor their smartphone use and foster positive parenting.\n"]