Across from Dorothea Buck’s hospital bed was a Christian message, a quote from the Gospel of Matthew written in large letters on the light green wall: “Come unto me, all ye that labor and are heavy laden, and I will give you rest.”

Ms. Buck, a pastor’s daughter in Hitler’s Germany, had been institutionalized at the Bethel psychiatric center, where she was diagnosed with schizophrenia after a psychotic episode in 1936. But in lieu of rest or gentle care, the 19-year-old faced treatment that she later likened to torture, presided over by nurses and physicians who rarely said a word.

There were 23-hour baths inside a canvas-covered tub, where her neck was held rigidly in place; unwanted injections of the sedative paraldehyde; and long stretches inside a cocoon of cold, wet sheets, wrapped so tight she was unable to move. She soon noticed that other female patients bore fresh scars along their lower abdomens, which doctors explained as the result of appendectomies.

Only after Ms. Buck was forced to undergo the procedure herself did she learn the truth from another patient. Her appendix had been untouched; instead, she had been sterilized under the Law for the Prevention of Offspring With Hereditary Diseases, a 1933 statute designed to help purify the “Aryan race.”

“I was distraught,” Ms. Buck said decades later , “because people who had had forced sterilizations were not allowed to attend secondary schools or schools providing higher education and were not allowed to marry a non-sterilized partner. I had to abandon my chosen profession as a kindergarten teacher.”

Ms. Buck was one of an estimated 300,000 to 400,000 victims of forced sterilization under Nazi rule, and she was described in recent years by the German press as one of its last survivors. The sterilization program targeted patients diagnosed with “feeblemindedness,” schizophrenia and epilepsy, among other conditions, and foreshadowed a wave of “euthanasia” killings, in which more than 200,000 people with mental and physical disabilities were systematically murdered.

While struggling to find work and make sense of a trauma that she described as “the most inhuman experience of my life,” Ms. Buck turned to ceramics and sculpture, crafting larger-than-life pieces that included a bronze depiction of a mother and child. She later traded art for activism, championing more humane therapeutic practices in Germany and co-founding support groups for victims of psychiatric abuses. “As long as there was no elementary humanity,” she explained, “art seemed less important.”

Ms. Buck was 102 when she died Oct. 9 at a care center in Hamburg. Her death was confirmed by Alexandra Pohlmeier, who chronicled Ms. Buck’s life in a 2009 documentary, “The Sky and Beyond.” She did not give a precise cause.

In books, essays, seminars and lectures, Ms. Buck espoused a message that was simple but profound, summarized by her friend and fellow activist Peter Lehmann in an article marking her 100th birthday. “Psychiatrists should talk with the people and not treat them as soulless objects,” he wrote, “and people with psychoses should try to make sense of their symptoms.”

Ms. Buck was institutionalized four more times after being released from Bethel, a Christian hospital in what is now the German city of Bielefeld. She was sometimes treated with electroshock therapy, and after her last psychotic episode, in 1959, was injected with a “high dosages of antipsychotic drugs,” which she called a “total dictatorship” that kept her from thinking and feeling.

Her turn toward mental-health advocacy came in the early 1960s, spurred partly by the trial of Holocaust organizer Adolf Eichmann and a broader reckoning with the crimes of the Nazi era. She wrote a play on the murder of people with disabilities, co-founded groups such as the Federal Organization of (Ex-)Users and Survivors of Psychiatry, and above all emphasized the importance of conversations between patients and physicians.

“I experienced the psychiatric system as being so inhuman because nobody spoke with us,” she said in a 2007 lecture at a conference organized by the World Psychiatric Association. “A person cannot be more devalued than to be considered unworthy or incapable of conversation.”

With Thomas Bock, now a professor of psychiatry and psychotherapy at the University of Hamburg, she developed a “trialogue” initiative in 1989, designed to encourage conversation between patients, friends and relatives, and mental-health professionals. Communication was also a theme she had pursued in her art, notably in a small sculpture of a mother and child that she gave in 2008 to the Charité, a leading European hospital affiliated with Humboldt University and the Free University of Berlin.

The sculpture “expresses the relationship between two people,” Ms. Buck wrote in a letter marking its donation, “which is still missing in today’s psychiatry due to the still inadequate conversations.” As she often told interviewers, “As long as we talk to each other, we don’t kill each other.”

Dorothea Sophie Buck-Zerchin was born in Naumburg, along the Saale River, on April 5, 1917. She was raised in Oldenburg, near Bremen, and on Wangerooge, one of the Frisian Islands in the North Sea, where her father had obtained a remote church posting amid tensions with the Nazi authorities. Her mother was a former kindergarten teacher.

Ms. Buck supported herself for more than a decade as an art teacher in Hamburg before shifting her focus to advocacy. Initially, she maintained a degree of anonymity, reportedly fearing that her psychiatric work would negatively impact her artistic career; her 1990 autobiography, “On the Trail of the Morning Star: Psychosis as Self-Discovery,” was published under the name Sophie Zerchin, an anagram of the German word for schizophrenia.

Before long, however, she was internationally recognized for her work. She received the Grand Cross of the Order of Merit, one of Germany’s highest honors, in 2008, and was recognized by the Hamburg Senate in 2017.

Ms. Buck has no immediate survivors. In recent years, she was still writing letters to politicians and campaigning on behalf of patients, including those who do not have the ability to consent to medical research. She established the Dorothea Buck Foundation in 2011 to support psychiatric patients who help others recover.

“As long as I’m still in good shape and can do something,” she told the German newspaper Die Welt, days before her 100th birthday, “I will continue this.”