According to Almughawim, which is an institute for preserving and publishing the books by the Bahraini top cleric, Sheikh Qassim arrived in Mashhad on Saturday.
Mashhad in northeast Iran is home to the holy shrine of the eighth Shia Imam.
The cleric arrived in Mashhad from Iraqi city of Najaf, where he visited the holy shrine of the first Shia Imam.
While in Najaf, Sheikh Qassim also visited Grand Ayatollah Ali al-Sistani as well as the Chairman of the Islamic Supreme Council of Iraq (ISCI) Ammar Hakim.
He is leading an opposition movement in Bahrain against Sunni al-Khalifa dynasty demanding an elected government in the Shia majority country.
He arrived in Najaf from Britain, where he received medical treatment after his health condition was deteriorated for being under house arrest since May 2017 by al-Khalifa regime.