Sendler died at a Warsaw hospital on Monday morning, her daughter, Janina Zgrzembska, said. She had been hospitalized since last month with pneumonia.
Sendler was serving as a social worker with the city's welfare department during World War II when she masterminded the risky rescue operations of Jewish children from the Warsaw Ghetto during Nazi Germany's brutal World War II occupation.
Records show that Sendler's team of some 20 people saved nearly 2,500 children from the Warsaw Ghetto between October 1940 and April 1943, when the Nazis burned the ghetto, shooting the residents or sending them to death camps.
"A great person has died - a person with a great heart, with great organizational talents, a person who always stood on the side of the weak," Marek Edelman, the last surviving leader of the 1943 Warsaw Ghetto uprising, told TVN24 television.
Under the pretext of inspecting the ghetto's sanitary conditions during a typhoid outbreak, Sendler and her assistants went inside in search of children who could be smuggled out and given a chance of survival by living as Catholics.
Babies and small children were smuggled out in ambulances and in trams, sometimes wrapped up as packages. Teenagers escaped by joining teams of workers forced to labor outside the ghetto. They were placed in families, orphanages, hospitals or convents.
In hopes of one day uniting the children with their families - most of whom perished in the Nazis' death camps - Sendler wrote the children's real names on slips of paper that she kept at home.
When German police came to arrest her in 1943, an assistant managed to hide the slips, which Sendler later buried in a jar under an apple tree in an associate's yard. Some 2,500 names were recorded.
"It took a true miracle to save a Jewish child," Elzbieta Ficowska, who was saved by Sendler's team as a baby in 1942, recalled in an AP interview in 2007. Mrs. Sendler saved not only us, but also our children and grandchildren and the generations to come.
Anyone caught helping Jews in Nazi-occupied Poland risked being summarily shot, along with family members - a fate Sendler only barely escaped herself after the 1943 raid by the Gestapo.
The Nazis took her to the Pawiak prison, which few left alive. She was tortured and was left with permanent scarring on her body - but she refused to betray her team.
"I kept silent. I preferred to die than to reveal our activity," she was quoted as saying in Anna Mieszkowska's biography, Mother of the Children of the Holocaust: The Story of Irena Sendler.
Zegota, an underground organization helping Jews, for which she worked at the time, paid a bribe to German guards to free her from the prison. Under a different name, she continued her work.
After World War II, Sendler worked as a social welfare official and director of vocational schools, continuing to assist some of the children she rescued.
Sendler's daughter once told the AP that during her childhood, the family house was always full of people asking for help, chiefly looking for their lost relatives.
In 1965, Sendler became one of the first Righteous Gentiles honored by the Yad Vashem Holocaust memorial in Jerusalem for wartime heroics. Poland's communist leaders at that time would not allow her to travel to Israel; she collected the award in 1983.
Despite the Yad Vashem honor, Sendler was largely forgotten in her homeland. Only in her final years, confined to a nursing home, did she finally become one of Poland's most respected figures, with President Lech Kaczynski and other politicians backing a campaign that put her name forward for the Nobel Peace Prize.
That effort came after her name was brought to the world's attention in 2000 by a group of U.S. schoolgirls from Uniontown, Kansas, who wrote a short play about her bravery based on historic records called Life in a Jar.
It went on to garner international attention, and has been performed more than 200 times in the United States, Canada and Poland.
Sendler, born Irena Krzyzanowska, was 2 years old when her family moved from Warsaw to nearby Otwock, where her father, a doctor, directed a spa hospital. He died of typhoid fever in 1917.
Sendler said she lived according to her father's teachings, arguing that people can be only divided into good or bad; their race, religion, nationality don't matter.
Before the war, she married Mieczyslaw Sendler, but they divorced when the war was over.
She then married another underground activist, Stefan Zgrzembski, and they had two sons and a daughter. One of the boys died a few days after birth. The second son, Adam, died of a heart failure in 1999.
Sendler is survived by her daughter and a granddaughter.
