The body of Christopher Butler, 27, from Southsea, was discovered at a flat in Waverley Road in the early hours of Friday.
A 23-year-old woman and a 33-year-old man, both from Portsmouth, are being held, Hampshire Constabulary said.
A post-mortem examination on Friday was inconclusive.
Two people have been arrested on suspicion of murder following the death of a man in Portsmouth on New Year's Day.