A new and distinct cultivar of Hydrangea plant named ‘HBA215908’, characterized by its upright and rounded plant habit; vigorous growth habit; moderately rapid growth rate; freely branching habit with strong, thick and sturdy stems; freely and uniformly flowering habit; mophead-type inflorescences with numerous red purple-colored sterile flowers; when plants are “blued”, sterile flowers are dark blue in color; and good postproduction longevity.