Police detained an Iranian national Sunday after raiding a rented room at Nasa Vegas Tower in Khlong Ton district, where another Iranian suspect Leila Rohani, also lived.
Rohani fled Bangkok and is now in Tehran, the capital of Iran.
Police with a search warrant raided room number 21434 on the 14th floor of the apartment building and detained an Iranian, identified as Madani Seyed Mehrded, 33.
The search came after police received information that Mehrded stayed in the room most of the time, while another Iranian provided him with hand-carried food and drink.
Police found no illegal items but seized a computer, a mobile phone and some other items for further inspection.
The authorities later learned that the suspect had overstayed his visa in Thailand, which expired on September 29 last year. He had entered Thailand on July 27, two months earlier.
Police said that they tracked Mehrded from telephone number information found in the SIM cards of two suspects already in police custody -- Saeid Moradi, 28, and Mohammad Hazaei, 42.
Moradi lost both legs from his own explosive device, and Hazaei was arrested at Suvarnabhumi Airport while attempting to board a plane bound for Malaysia shortly after triple explosions rocked Bangkok on February 14
According to police, the call logs on the SIM cards showed Mehrded had regularly communicated with both suspects by mobile phone.
On February 14, he was allegedly seen in front of the Israeli embassy on Asoke Road.
Police said an Iranian working as a chef at a Soi Nana hotel supplied Mehrded with food and drink. The chef lived in the same apartment complex.
Mehrded was in the room with foreign women. All were taken for questioning.
The fifth suspect is a Middle Eastern man who was captured by CCTV footage walking away from the rented house where the first bomb apparently exploded by accident.