Altmann died Monday at her home in the Cheviot Hills area of Los Angeles after a long illness, said E. Randol Schoenberg, her friend and attorney.
Altmann was already in her 80s in 1998 when she and Schoenberg began a legal fight with the Austrian government over the paintings, which included a world-famous gold-encrusted picture of her aunt, the "Portrait of Adele Bloch-Bauer."
The 1907 masterpiece hung in the Austrian Gallery Belvedere in Vienna. The Austrian government contended that Altmann's aunt, who died in 1925, had willed it to the Austrian national gallery.
However, Schoenberg argued the claim was invalid, and the family's art collection was plundered more than a decade later by the Nazis.
"These were private paintings. They were in her aunt and uncle's home until the Nazis came in and took everything," Schoenberg said.
Altmann's lawsuit was given little chance of success at the time.
"Slim and none, I guess, was the legal term," Schoenberg said. "When you sue a foreign country, it's generally not possible to do it and I found a very small exception that really had not been used before."
In 2004, the U.S. Supreme Court ruled that Altmann's suit could proceed. In 2006, an Austrian mediation panel awarded the five paintings to Altmann and four other heirs, ending the nearly eight-year legal battle.
The "Portrait of Adele Bloch-Bauer" was purchased for $135 million that year — a record price for a painting at the time — and is now in the Neue Galerie in New York.
The other four paintings were auctioned for $192.7 million later that year and are now in private collections, Schoenberg said.
The Vienna group Jewish Community called Altmann a "fearless fighter for justice."
"For us, Maria Altmann's passing is a painful loss," said a statement signed by Jewish Community President Ariel Muzicant and Executive Director Erika Jakubovits. "Her positive attitude toward life, coupled with a strong will, led her to victory, despite much hostility along the way."
Mark A. Rothman, executive director of the Los Angeles Museum of the Holocaust, said Altmann's case was one of many about the unfinished business of the Holocaust.
"The restituting of property and assets is an important part of trying to resolve the incalculable injustices perpetrated by the Nazis," he said
Altmann made a large contribution to the museum, and Schoenberg, who donated $6 million, is president.
Altmann, who was born Maria Viktoria Bloch-Bauer in Vienna, came from a wealthy Jewish family that was stripped of its businesses, home and other wealth after Germany annexed Austria in 1938.
Maria Altmann was a 21-year-old newlywed when the Gestapo seized her husband, Fritz, and sent him to the Dachau concentration camp in an effort to force his brother to turn over a textile factory.
The Gestapo flew her to Berlin to seal the deal for her husband's release, Schoenberg said. After both were free, they fled to Holland and later moved to the United States.
Her aunt's husband fled to Switzerland, and his home and art collection were seized. He died in Zurich in November 1945 after failing to recover his wife's portrait and other paintings.
In 1998, an Austrian investigative journalist revealed details of the Nazi plundering of Jewish-owned artworks, and Austria enacted a law requiring the return of looted art.
Altmann, then an elderly grandmother, began her fight to recover her family's property.
"She was not at all obsessed by it. It did not become a quest for her," but she pursued it with the same resolve she had used in dealing with the Nazis all those decades ago, Schoenberg said.
Altmann is survived by her sons, Charles Altmann of Los Angeles; James Altmann of Agoura Hills; and Peter Altmann of Puget Sound, Wash.; a daughter, Margie Cain of Solana Beach, six grandchildren and two great-grandchildren.
Associated Press writer Veronika Oleksyn in Vienna contributed to this report.