The incident, involving a Stagecoach single-decker bus and pedestrian, occurred at around 6.47pm on London Road.
Emergency services attended and the road was closed, however the pedestrian, a 64-year-old man from Portsmouth, was pronounced dead at the scene.
His family have been informed and are being supported by officers at this time.
Stagecoach, and the driver, are assisting with enquiries, while the investigation continues to understand the exact circumstances of the collision.
Officers investigating are keen to hear from any witnesses they have not already spoken with.
They are particularly keen to hear from anyone who has dash-cam footage or any local businesses who may have relevant CCTV footage in the moments leading up to the incident.
Anyone who can assist with enquiries should call 101, quoting the reference 44220159148.
