Globally, the human respiratory syncytial virus (RSV) is one of the major causes of lower respiratory tract infections (LRTIs) in children. The scarcity of complete genome data limits our understanding of RSV spatiotemporal distribution, evolution, and viral variant emergence. Nasopharyngeal samples collected from hospitalized pediatric patients from Buenos Aires tested positive for RSV LRTI during four consecutive outbreaks (2014-2017) were randomly subsampled for RSV complete genome sequencing. Phylodynamic studies and viral population characterization of genomic variability, diversity, and migration of viruses to and from Argentina during the study period were performed. Our sequencing effort resulted in one of the largest collections of RSV genomes from a given location (141 RSV-A and 135 RSV-B) published so far. RSV-B was dominant during the 2014-2016 outbreaks (60 per cent of cases) but was abruptly replaced by RSV-A in 2017, with RSV-A accounting for 90 per cent of sequenced samples. A significant decrease in RSV genomic diversity-represented by both a reduction in genetic lineages detected and the predominance of viral variants defined by signature amino acids-was observed in Buenos Aires in 2016, the year prior to the RSV subgroup predominance replacement. Multiple introductions to Buenos Aires were detected, some with persistent detection over seasons, and also, RSV was observed to migrate from Buenos Aires to other countries. Our results suggest that the decrease in viral diversity may have allowed the dramatic predominance switch from RSV-B to RSV-A in 2017. The immune pressure generated against circulating viruses with limited diversity during a given outbreak may have created a fertile ground for an antigenically divergent RSV variant to be introduced and successfully spread in the subsequent outbreak. Overall, our RSV genomic analysis of intra- and inter-outbreak diversity provides an opportunity to better understand the epochal evolutionary dynamics of RSV.