Haplogroup R1a, or haplogroup R-M420, is a human Y-chromosome DNA haplogroup which is distributed in a large region in Eurasia, extending from Scandinavia and Central Europe to Central Asia, southern Siberia and South Asia.

In human genetics, a human Y-chromosome DNA haplogroup is a haplogroup defined by mutations in the non-recombining portions of DNA from the male-specific Y chromosome. Many people within a haplogroup share similar numbers of short tandem repeats (STRs) and types of mutations called single-nucleotide polymorphisms (SNPs).