Troops surrounded the home of Mahmoud al-Rahami - secretary-general of the Palestinian Legislative Council – and detained him in broad daylight, his sister Yageen said.
An army spokeswoman confirmed that Israeli soldiers and officers from the Shin Bet internal security service had seized the member of parliament in Ram Allah.
Almost all of Hamas's leaders in the West Bank are now in Israeli custody.
The crackdown on the group's officials began after the capture of an Israeli soldier by gunmen in the Gaza Strip on June 25. The military wing of Hamas is one of three groups believed to be behind the operation.
Al-Ramahi, a trained doctor, is the fourth highest ranking member of the parliament and is responsible for many administrative and procedural matters.
Dozens of officials seized
Early on Saturday, troops arrested Nasser Shaer, the deputy prime minister, at a home where he had been hiding in Ram Allah. He was the most senior Hamas official to be detained yet.
Five cabinet ministers and more than two dozen MPs are being held by Israel. Four other members of the cabinet have been released.
Mushir Masri, a Hamas MP in Gaza, called the arrest "cheap Israeli extortion that runs against all international laws".
He told The Associated Press news agency: "We hold Israel responsible for all the serious consequences if it insists on kidnapping lawmakers and ministers".