Ahmed el-Haj Ali, 66, a leading Hamas cleric from Nablus in the West Bank, said he was released on Wednesday after five months in jail under a so-called administrative detention order.
The army made no comment on his release. Israel has ruled out contacts with Hamas unless it renounces violence and drops its charter call for the Jewish state's destruction.
"Israel found no further justification to keep me in jail," Ali told Reuters.
The new Palestinian parliament will be sworn in on Saturday.
Hamas took 74 of the parliament's 132 seats in a surprise election victory, trouncing the long-dominant Fatah of Mahmoud Abbas, the Palestinian president. Ali won more votes than any other candidate in Nablus.
Israel has held thousands of Palestinians without trial during an uprising and says such detentions help to prevent attacks. Others have been convicted of helping to carry out attacks.
With Ali's release, 11 of Hamas's new parliament members remain behind bars. There are three from Fatah, including Marwan Barghouthi, the leader of the uprising, serving five life terms.
Hamas has carried out nearly 60 suicide bombings against Israelis since a Palestinian uprising began in 2000, but has largely adhered to a truce declared last year.