Police in the city said the 60-year-old man and a 45-year-old woman were injured int the blast at the Varby Gard metro station in the Huddinge district.
The man was rushed to hospital in a serious condition but later died from multiple injuries sustained in the blast.
Police confirmed he had died in a statement on their website.
The condition of of the second blast victim, believed to be the man’s wife, is unknown.
Officers said initial reports suggested the man was injured when he picked up an object, possibly a hand grenade, which then exploded in his hands.
They said there was no evidence to suggest the bast was terror-related.
A police spokesman said: “Police patrols have encountered a person injured in what is believed to be an explosion.
“After talking to a person who was in the company of the injured party, it appears that the explosion was caused by an object from the ground.”
The spokesman said the bomb squad was on its way to the scene to investigate the cause of the explosion.
Police seal off the Stockholm metro station blast scene
Stockholm fire department spokesman Lars-Ake Stevelind told Sveriges Radio: “Someone has used explosives. We do not know more than that.”
Police cordoned off the metro station and launched a search of the area. A section of the metro line was shut.
Varby Gard station is on the Stockholm metro’s T13 line, which links several southern and northern suburbs to the city centre.
Huddinge resident Charlotte Bergstrom said her flat shook when the explosion occurred.
She said: “It was the loudest bang.”
Local shopkeeper Abdullah Ozmen said he heard the blast and then saw the injured pair on the floor.
He said: “We heard the explosion. When we went out we saw people lying on the ground.”