Iranian security forces on Tuesday raided Tajzadeh's daughter's home and arrested him without providing any explanation, former fellow inmate Mehdi Mahmoudian said on X.
"The re-arrest of Tajzadeh reflects the Islamic Republic’s blatant disregard for human dignity and justice," the State Department said in a statement on its Persian-language X account.
"It also shows that, for the Islamic Republic, suppressing dissent takes priority over addressing Iran’s deeper crises."
Tajzadeh is a reformist politician who served as deputy interior minister during the presidency of Mohammad Khatami from 1997 to 2005.
He has been imprisoned for 10 of the last 16 years, currently on charges including acting against the state, spreading falsehoods and propaganda.
Before being re-arrested, Tajzadeh attended the funeral of his brother and met several activists including Nobel Peace Prize laureate Narges Mohammadi in a move the State Department hailed as “a symbol of the resistance and courage of the Islamic Republic’s political dissidents.”
Mohammadi said on Tuesday that there was no prospect for reforming the country's Islamic theocracy and its downfall was assured.
“Reform has been dead for years. The time for reforms has long passed. The real main struggle is between the realistic survivalists and those seeking the end of religious despotic regime,” Mohammadi posted on X.
In July, Tajzadeh warned Supreme Leader Ali Khamenei to pivot or resign.
"In this critical situation, Mr. Khamenei has no option but to apologize to the Iranian people and accept fundamental reforms in line with national demands, including by forming a constituent assembly based on completely free and fair elections," he said, "or to resign and step down."
In recent years, the pursuit of reform has shifted toward regime change, as seen in the 2017-18 and 2022 Woman, Life, Freedom uprisings, with many people viewing the system as irreformable.