The man identified as Mohamed Ibrahim Ifthikar has been arrested at Dematagoda area in a suburb outside Colombo, Ada Derana news portal reported.
A police spokesperson said, as quoted by the media outlet, that policemen had found blue sapphires, as well as Sri Lankan and foreign money worth over Rs 10 million ($56,984) in his house in Dematagoda.
Another brother of the suicide attacker has allegedly attacked a restaurant at the luxury Cinnamon Grand hotel.
Earlier in the day, local media reported that Sri Lankan police had arrested two main suspects in the deadly bombings, which rocked the country last Sunday.
A string of deadly bombings hit Sri Lanka on Easter Sunday leaving hundreds of people killed and injured nationwide. Over 100 suspects have been detained since the country launched a massive anti-terrorist operation and investigation into the attacks.
Daesh* has reportedly claimed responsibility for the attacks.
* Daesh (ISIL/ISIS/IS/Islamic State) is a terrorist organisation banned in Russia