Few disorders appear to be more challenging for parents than autism spectrum disorder (ASD). Little is known about the extent to which parenting stress experienced by parents of children with ASD affects or is affected by marital quality. We examined daily spillover between level of parenting stress and marital interactions in a sample of 176 married couples (89.4% Caucasian, non-Hispanic) who have a child with ASD (5-12 years of age, 85% male) via a 14-day daily diary approach. On each day of the daily diary, parents individually reported on 8 positive and 8 negative marital interactions and their level of parenting stress. Dyadic multilevel modeling analyses using hierarchical linear modeling were conducted to examine same-day and lagged-effect associations between number of positive and negative marital interactions and level of parenting stress. Having a day with a higher number of negative marital interactions was associated with a higher level of parenting stress for both mothers and fathers of children with ASD. Having a day with fewer positive marital interactions was associated with having a more stressful parenting day for mothers of children with ASD. Same-day spillover was moderated by parent gender and the functioning of the child with ASD. Spillover flowed bidirectionally for mothers of children with ASD. Helping parents of children with ASD find ways to engage in positive marital interactions on stressful parenting days and avoid having negative affect, tension, and behaviors stemming from negative marital interactions spill into parenting experiences are important intervention targets.