The comments by Mahmoud al-Zahar contrasted with remarks by Ismail Haniyeh, who serves as prime minister of a Hamas-led government dismissed by Palestinian President Mahmoud Abbas.
Haniyeh said earlier this week that Hamas had no intention of repeating its Gaza takeover in the Israeli-occupied West Bank where Abbas's secular Fatah faction remains dominant.
"Israel says the party in Ramallah (Fatah) serves Israel, and if Israel quits the West Bank, Hamas will take it over. And we say this is true," Zahar said at a rally for Hamas supporters in Jabalya refugee camp in northern Gaza.
Hamas Islamists took over the Gaza Strip after routing forces loyal to the Western-backed Abbas. Israel and the United States are trying to bolster Abbas, who dismissed the Hamas-led government and formed his own cabinet in the West Bank.
Israel declared the Hamas-controlled Gaza Strip an "enemy entity" and closed its borders with the coastal territory, allowing only humanitarian aid to enter.
"We say to those in the West Bank take a lesson from what happened in Gaza," Zahar said, an indirect reference to the fierce violence that led to Hamas's takeover of the coastal strip.
Israel has warned that Hamas would be poised to take over the territory were it to withdraw its forces.