The timing of seasonal life-history events is assumed to evolve to synchronize life cycles with the availability of resources. Temporal variation in breeding time can have severe fitness consequences for the offspring, but the interplay between adult reproductive decisions and offspring phenotypes remains poorly understood. Transgenerational plasticity (TGP) is a potential mechanism allowing rapid responses to environmental change. Here, we investigated if experimentally delayed breeding induces TGP in larval life-history traits in the moor frog (Rana arvalis). We found clear evidence of TGP in response to changes in breeding phenology: delayed breeding increased offspring development and growth rates in the absence of external cues. This constitutes the first unequivocal evidence for TGP in response to changes in breeding phenology in vertebrates. TGP can play an important role in adjusting offspring life-history strategies to the environment they are most likely to encounter, and may constitute an important mechanism for coping with climate change.