Siyam was born in the al-Shati refugee camp near Gaza City and later earned a diploma in sciences and math, followed by a bachelor's degree in Islamic Education from Al-Quds University.
It is not known when Siyam first joined Hamas, but he is believed to have been one of the organization's earliestmembers. He was arrested five times by the Israel Defense Forces during the First Intifada and in 1992, Israel expelled him along with a number of other high-ranking Hamas members to south Lebanon. He returned to Gaza in 1995.
After the assassinatons of Hamas leaders Ahmed Yassin and Abd al-Aziz Rantisi in 2004, Siyam became a a member of the Hamas "collective leadership." According to the Israeli daily Ha'aretz, Siyam has been put in charge of contacts between Hamas and other political groups in Gaza, including Fatah and Palestinian Islamic Jihad.
After the Palestinian parliamentary elections in 2006, Seyam was appointed Interior Minister of the Palestinian National Authority.
On January 15, 2009, Siyam was killed in an IDF air strike on his brother's home in the Jabalia section of Gaza City. Two other Hamas leaders, Saleb Abu Sharkh and Mahmoud Abu Watfah, were also killed in the targeted strike.