Neamatallah Hisham - wife of prominent human rights lawyer Mohamed al-Baqer - was "arrested, had her phone confiscated and taken to an unknown location" by security forces in Cairo early on Monday morning, the Egyptian Initiative for Personal Rights said.
The arrest came "a few hours after she visited her husband and posted about ill-treatment he was subjected to by prison staff," Lawlor wrote on Twitter.
Hisham's posts said Baqer had been "beaten," "gagged," "made to stand in his underwear for two days," and "kept in solitary" confinement last week.
UN Special Rapporteur on Human Rights Defenders Mary Lawlor - who regularly calls attention to Baqer's case, along with that of his imprisoned client Alaa Abdel Fattah - called the news "disturbing."
She said prison authorities had targeted Baqer and his cellmates - Mohamed "Oxygen" Ibrahim, Ahmed Douma, Hamid Siddiq and Mohamed al-Qassas - for defending an elderly inmate from being badly treated.
Baqer is serving a four-year jail term for "broadcasting false news," after being arrested in 2019 while attending an interrogation of his client, prominent dissident Alaa Abdel Fattah, whose case made headlines last year as Cairo came under fire for its rights record.
Pro-democracy blogger Abdel Fattah is currently serving a five-year sentence for "spreading false news" by sharing another user's Facebook post about police brutality.
Rights groups say there are around 60,000 political prisoners in Egypt, many facing brutal conditions in overcrowded cells.
Egypt has in the past year granted pardons and announced a "national dialogue" that has been repeatedly delayed and met with skepticism.
According to Amnesty International, 895 political prisoners were released between April and December last year, but close to triple that number were detained.