Breastfeeding practices during hospitalisation may influence subsequent breastfeeding practices; however, this association has not been well studied in Japan. Therefore, we aimed to examine the association between exclusive breastfeeding (EBF) during hospitalisation and that under 6 months and describe the change in breastfeeding patterns from the first to the sixth month based on the breastfeeding status during hospitalisation. This nationwide cross-sectional internet survey conducted in Japan included 1,433 postpartum women of < 6 months who underwent live singleton deliveries between January 2021 and August 2021. Multivariate Poisson regression was used to analyse the association of first day EBF (24 h after birth) and the first 7 d of EBF, a proxy for the hospitalisation period, with a 24-h recall of EBF before the survey. We described the proportion of breastfeeding practices per age group under 6 months. The rates of EBF during the first day and the first 7 d were 29.7% and 10.0%, respectively. The EBF during the first-day group and the first 7-d group showed significantly higher prevalence ratios of 24-h recall EBF under 6 months of age than the non-EBF groups. The area graphs showed that the rate of EBF was the lowest in the first month of age and gradually increased over time until weaning was initiated. This rate was higher among infants exclusively breastfed during the first 7 d than among those exclusively breastfed on the first day. In conclusion, EBF during hospitalisation was significantly associated with subsequent EBF practice for < 6 months. However, 90% of the infants were supplemented with milk rather than breast milk during hospitalisation. Strengthening breastfeeding support during hospitalisation and after discharge may increase the rate of EBF in children under 6 months, and more mothers and their infants will benefit from breastfeeding.