The 55-year-old pediatrician, who was wounded in an Israeli missile strike Tuesday, has issued fiery calls for deadly attacks on Israelis and is an outspoken critic of any Hamas cease-fire.
In 1987, Rantisi was one of six men who founded Hamas, which became one of the region's largest militant Islamic groups. Its ideology calls for a Muslim Middle East without an independent Jewish state -- Israel (search).
Hamas pioneered homicide bombings in Israel in the early 1990s. Over the past 32 months, the group has killed hundreds of Israelis with homicide bombings, remote-controlled bombs and rifle attacks.
The fourth of 12 children, Rantisi was born Oct. 23, 1947, in Yibnah, now the Israeli town of Yavneh.
When he was six months old, his parents fled fighting around Israel's 1948 creation and took him to the Gaza Strip, then part of Egypt, where they settled in the impoverished Khan Younis refugee camp.
He traveled to Alexandria, Egypt, after high school and studied pediatrics, later working at a hospital in Gaza and earning a degree in medicine.
After the founding of Hamas, Rantisi was the first of the group's leaders to be arrested by Israel. In and out of Israeli custody several times, he spent more than seven years behind bars altogether.
During one confinement, he shared a cell with Hamas' spiritual leader Sheik Ahmed Yassin and memorized the Quran -- 600 pages in most standard editions.
In prison, he used empty milk, toothbrush and cigarette cartons -- though he's a nonsmoker -- to construct a model of Jerusalem's Al Aqsa Mosque, which still sits, colorfully painted, on a table in his Gaza City living room.
Israel sent Rantisi and more than 400 other Hamas members into temporary exile in Lebanon in 1992 after the killing of an Israeli soldier. Rantisi became internationally known there, using his fluent English to speak for the deportees.
Appearing at the head of Gaza street marches in his gold-framed tinted glasses, Rantisi now often serves as a spokesman for Hamas. He welcomes journalists to his apartment, setting out sweets and coffee for his visitors.
Hamas considers Rantisi a political leader. But Israeli security sources say he sets the policy of operations against Israel and is responsible for recruiting Arab Israelis for attacks. Israeli government spokesman Ranaan Gissin said Rantisi masterminded an attack Sunday that killed four Israeli soldiers.
Rantisi has six children and 10 grandchildren. He has written poetry for one of them, a girl named Assma.
On a personal Web site he set up, Rantisi posted a poem. One line exults: "The mockingbird burst into song. He was drawn by her magic when he saw her."
In an introduction, he said he was inspired when his first granddaughter visited him in an Israeli prison. "She kissed me and it was one of the wonderful moments in my life," he wrote.