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.

Topal, a 29-year-old Turkey midfielder with Fenerbahce, was driving home with a team-mate after training when the car was attacked.
The club condemned the incident as an "armed terrorist act".
The reason for the attack is unclear, but Fenerbahce's team bus was shot at earlier this year and the driver was wounded in the city of Trabzon.
The bus had been driving over a viaduct and a club official was forced to intervene to bring the vehicle to a halt.
The club said at the time that the attack represented the "peak point of hostility" towards it.
Turkey's top-flight Super League was suspended for a week and President Recep Tayyip Erdogan, seen as a Fenerbahce supporter, called a meeting of the league's 18 club captains.
No motive has been established for Tuesday's attack.
BBC Turkish reporter Emre Bal said that although there was an intense rivalry between fans of Fenerbahce and another Istanbul club, Galatasaray, Mehmet Topal was a popular player.
Former Fenerbahce footballer Kennet Andersson told the BBC that opposing fans regularly threw objects including fireworks at team buses, but the latest violence was different. "It sounds much worse now and it shouldn't be like that," he said.
Immediately after the shooting, Topal pulled his car off the road in Istanbul's eastern Sancaktepe district and called for police help.
Topal's team-mate Uygar Mert Zeybek was also unhurt.
Pictures released by the club showed bullet damage on the passenger side of the widescreen, although the bullet did not appear to have pierced the glass.
The club had earlier published pictures on its website of recent signing Robin Van Persie taking part in training.
Fenerbahce were knocked out of the qualifying rounds of the Uefa Champions League by Shakhtar Donetsk last week.
Two people have been arrested on suspicion of murder following the death of a man in Portsmouth on New Year's Day.
[NEXT_CONCEPT]
Turkish footballer Mehmet Topal has escaped unharmed after gunmen fired on his bulletproof car in Istanbul.