Robert Venturi, 93, a Philadelphia architect who radically altered the way the profession thinks about Modernism, history, popular culture, neon signs, and roadside architecture, died Wednesday at his West Mount Airy home. He had been suffering from complications of Alzheimer's disease and was in hospice care, according to a family spokeswoman.

Mr. Venturi's influence came both from his stunningly unorthodox buildings, like Guild House on Spring Garden Street, and from his groundbreaking books on architectural history.

His most famous essay, Complexity and Contradiction in Architecture, is considered one of the most important books — on any subject — of the 20th century. Published in 1966, the treatise, which Mr. Venturi referred to as his "gentle manifesto," helped liberate a generation of architects from the demanding ideology of Modernism. That book, along with Learning From Las Vegas in 1972, which he co-authored with his wife, Denise Scott Brown, and Steven Izenour, became a pop culture touchstone.

For someone who liked to think of himself as a bad boy of architecture, Mr. Venturi was an unlikely rebel. His father owned a produce store at 1430 South St. His mother was a Socialist and pacifist.  He was raised as a Quaker and sent to Episcopal Academy, where he perfected a tweedy style of dress that included bow ties and horn-rimmed glasses. The uniform rarely varied, even during the bell-bottom days of the 1970s.

Mr. Venturi spent years teaching at University of Pennsylvania and other Ivy League schools, and often spoke in a mumbly academese. Even though he championed "vulgar" and vernacular buildings, like roadside drive-ins and Vegas casinos, you had to be steeped in the academic world to appreciate some of his architectural jokes and references. One reason he moved his architecture firm from 16th and Pine Streets in Center City to Manayunk in 1980 is because it would be located on Main Street. He had already turned the phrase, "Main Street is almost alright," into a famous rallying cry to celebrate the unlikely juxtapositions found on American streets.

David Brownlee, a University of Pennsylvania professor and an expert on Mr. Venturi's work, believes his importance extends beyond architecture. During the cultural and aesthetic upheavals of the '60s and '70s, when people such as Andy Warhol were challenging conventional approaches to art-making, Mr. Venturi "literally gave us a vocabulary to talk about change."

His buildings were often the architectural equivalent of works by those pop artists. Long before architect Frank Gehry incorporated chain-link fencing into his Santa Monica, Calif., home, Mr. Venturi used the material at Guild House. He topped the building with a giant, nonfunctional TV antenna, treating it like the point of a steeple. He loved the idea of incorporating neon the way Renaissance architects decorated their buildings with stone carvings.

Mr. Venturi is often credited with launching the Postmodernist movement in architecture, which sought to bring back references to traditional architecture, like columns and arches. While his buildings incorporated plenty of oversize, almost cartoonish, classical elements, he hated being labeled a postmodernist, said William Whitaker, who runs Penn's Architectural Archive. He preferred to think of himself as a "mannerist" who "enriched the Modernist mission" by adding signs and symbols.

Not everyone bought that explanation, and Mr. Venturi was frequently blamed for leading architecture down a kitschy path that resulted in strip malls with pediments and columns. The writer Tom Wolfe complained that Mr. Venturi pretended to be a populist but was actually an elitist.

After completing an undergraduate and a master's degree at Princeton, Mr. Venturi went to work for the Modernist architect Eero Saarinen in Michigan. But after just two years, he returned home to help his ailing father run the produce business. Mr. Venturi continued to oversee the South Street store even after his architecture practice began to thrive.

He had been gaining notice for several small commercial projects when his mother asked him to design a house in Chestnut Hill. Completed in 1964, the Vanna Venturi House looked like something a child might draw but was actually filled with historical references and inside jokes. It split the architecture world, much as Mr. Venturi split its peaked facade with a deep cleft. But it made him famous. Featured on a U.S. postage stamp in 2005, it was listed on Philadelphia's historic register in 2016. It has become a pilgrimage site for students of architecture.

It is nearly impossible to separate Mr. Venturi's accomplishments from the work of Scott Brown. The two began collaborating in 1962, and many believe that it was Scott Brown who introduced him to the literary and artistic debates. They married in 1967 and worked side by side until they formally dissolved their firm in 2012.

Even so, it was Mr. Venturi who usually got all the credit. When he won the prestigious Pritzker Architecture Prize in 1991, Scott Brown was mentioned only in passing, leading to tension in their partnership. Only in recent years has their work been recognized as a true collaboration. In 2015, the American Institute of Architects awarded the couple its Gold Medal, partly in an attempt to redress past snubs of Scott Brown.

Mr. Venturi and Scott Brown came into their own when Philadelphia was a hotbed of architectural innovation. He formed a close friendship with Louis Kahn, another important 20th century architect, as well as George Howe, Oscar Stonorov, and Romaldo Giurgola.

Philadelphia, a city that is both innovative and resistant to change, is probably the only place in America that could have produced a figure like Mr. Venturi. His work combined the best aspects of his hometown to open up the world to a new way of seeing buildings.

He is survived by his wife and a son, Jim.

Services will be private, but a public memorial is being planned.