Voicing a position that had been widely expected, Khalid al-Batsh, a leader of the group, said: "Islamic Jihad will not join the coming cabinet.
"If the government will have an agenda of resistance, we will support it," he said.
Batsh said any long-term ceasefire with Israel would be useless and Islamic Jihad "rejects it completely".
In a BBC interview, Khalid Mishaal, the Hamas politburo chief, said Hamas could give a long-term truce but only if Israel withdrew from the occupied West Bank, recognised a right of return for Palestinian refugees and dismantled all settlements.
Israel has rejected those conditions, first voiced by Shaikh Ahmad Yasin, the Hamas founder it assassinated in 2004.
Hamas and Islamic Jihad have previously stated they are committed to Israel's destruction.
Hamas, which swept to victory in a 25 January parliamentary election, has largely abided by a ceasefire Mahmoud Abbas, the Palestinian president, declared with Israel a year ago.
Islamic Jihad accepted the truce but later pronounced it dead, carrying out several bombings in retaliation for Israeli attacks.