West Midands Police said the suspect, aged 25, was arrested at 5.05pm on Saturday in relation to “a further act of terrorism”.
A police spokesman said: “This arrest is in connection with the murder of Mohammed Saleem in Small Heath, Birmingham, on Monday, April 29.”
Mr Saleem died after being stabbed in the back in Green Lane, Small Heath, as he made his way home from a mosque.
Two Ukrainian nationals, aged 25 and 22, were detained by West Midlands Police in Small Heath on Thursday by detectives investigating explosions at mosques in Walsall, Wolverhampton and Tipton between June 22 and July 12.
The family of Mr Saleem have been informed of the latest development in the inquiry, the police spokesman added.
Assistant Chief Constable Marcus Beale, who is leading the investigation, said in a short statement: “The murder of Mohammed Saleem now forms part of the wider West Midlands Counter Terrorism Unit Investigation.”
The stabbing of Mr Saleem, a highly-regarded and well-known figure in the local community, was described by detectives at the time as a “despicable” attack on a defenceless pensioner.
On Saturday, counter-terrorism officers were granted extra time to question the Ukrainian nationals in connection with the explosions. A sitting of Westminster Magistrates Court granted officers up to seven days to question the men, who were initially detained on suspicion of being involved in the commission, preparation or instigation of an act of terrorism.
The Labour MP for Birmingham Ladywood, Shabana Mahmood, said: “I welcome the news that an arrest has been made in connection with the murder of Mr Saleem, a much-loved member of our community.
“Although nothing can bring him back, the fact that a significant step has been taken towards getting justice for Mr Saleem will be a comfort to his family and the community in Small Heath.
“It is shocking that Mr Saleem’s brutal murder may have occurred as an act of terrorism.”