Understanding the variability of the Atlantic Meridional Overturning Circulation on different timescales is essential for better predictions of our evolving climate. The newly updated time series (August 2014 to June 2020) from the Overturning in the Subpolar North Atlantic Program array allows us to examine the observation-based seasonality of the subpolar overturning, and meridional heat and freshwater transports for the first time. The overturning peaks in late spring and reaches a minimum in early winter, with a peak-to-trough range of 9.0 Sv. The overturning seasonal timing can be explained by winter transformation and the export of dense water, modulated by a seasonally-varying Ekman transport. Furthermore, over 55% of the total meridional freshwater transport variability can be explained by its seasonality, largely owing to overturning dynamics. Our results highlight the importance of seasonality in subpolar overturning variability and provide the first observational evidence of its variability on seasonal and longer timescales.