"The jail sentence on September 3 against Iranian human rights defender Parvin Ardalan gives rise to concern," Bildt said in a statement, adding that "the claim that Ardalan poses a threat to Iran's national security lacks credibility."
He added that the move revealed Tehran's "deteriorated respect for human rights."
Ardalan, who won the 2007 Olof Palme Prize but was prevented by Tehran from attending the ceremony in Sweden last March, was sentenced to six months prison along with three other women's rights activists.
The 41-year-old campaigner was detained along with 70 other people for a June 2006 demonstration in Tehran square demanding equal rights for women on divorce, inheritance and child custody.
Ardalan and the three others sentenced this week are active members of an initiative that seeks to change Iran's Sharia-based laws for women by collecting one million signatures.
They were sentenced over articles on the "Change for Equality" and "Zanestan" websites, according to their lawyer, Nobel peace laureate Shirin Ebadi.
"I see the sentence as yet another expression of the deteriorated respect for human rights in Iran," Bildt said, adding that he was concerned over "an obvious negative development in recent months."
He pointed out that "the number of executions has dramatically increased and more have taken place in public."
"At least five people have since June been executed for crimes committed when they were minors (and) the list of people who risk suffering the same fate is long," he added.