Frank Heyling Furness (November 12, 1839 – June 27, 1912) was an American architect of the Victorian era. He designed more than 600 buildings, most in the Philadelphia area, and is remembered for his diverse, muscular, often inordinately scaled buildings, and for his influence on the Chicago-based architect Louis Sullivan. Furness also received a Medal of Honor for bravery during the Civil War.

Toward the end of his life, his bold style fell out of fashion, and many of his significant works were demolished in the 20th century. Among his most important surviving buildings are the University of Pennsylvania Library, now the Fisher Fine Arts Library, the Pennsylvania Academy of the Fine Arts, and the First Unitarian Church of Philadelphia, all in Philadelphia, and the Baldwin School Residence Hall in Bryn Mawr.

Furness was born in Philadelphia on November 12, 1839. His father, William Henry Furness, was a prominent Unitarian minister and abolitionist, and his brother, Horace Howard Furness, became America's outstanding Shakespeare scholar. Frank, however, did not attend a university and apparently did not travel to Europe. He began his architectural training in the office of John Fraser, Philadelphia, in the 1850s. He attended the École des Beaux-Arts-inspired atelier of Richard Morris Hunt in New York City, from 1859 to 1861, and again in 1865, following his military service. Furness considered himself Hunt's apprentice and was influenced by Hunt's dynamic personality and accomplished, elegant buildings. He was also influenced by the architectural concepts of the French engineer Viollet-le-Duc and the British critic John Ruskin.

Furness's first commission, Germantown Unitarian Church (1866–67, demolished ca. 1928), was a solo effort, but in 1867, he formed a partnership with Fraser, his former teacher, and George Hewitt, who had worked in the office of John Notman. The trio lasted less than five years, and its major commissions were Rodef Shalom Synagogue (1868–69, demolished) and the Lutheran Church of the Holy Communion (1870–75, demolished). In 1897, Furness designed an addition to the Philadelphia Savings Fund Society (PSFS) 1869 building which has now been incorporated into the St. James, a high-rise luxury apartment complex in the city’s Washington Square neighborhood.

Following Fraser's move to Washington, D.C., to become supervising architect for the U.S. Treasury Department, the two younger men formed a partnership in 1871, and soon won the design competition for the Pennsylvania Academy of the Fine Arts (1871–76). Louis Sullivan worked briefly as a draftsman for Furness & Hewitt (June – November 1873), and his later use of organic decorative motifs can be traced, at least in part, to Furness. By the beginning of 1876, Furness had broken with Hewitt, and the firm carried only his name. Hewitt and his brother William formed their own firm, G.W. & W.D. Hewitt, and became Furness's biggest competitor. In 1881, Furness promoted his chief draftsman, Allen Evans, to partner (Furness & Evans); and, in 1886, did the same for four other long-time employees. The firm continued under the name Furness, Evans & Company as late as 1932, two decades after its founder's death.

Furness was one of the most highly paid architects of his era, and a founder of the Philadelphia Chapter of the American Institute of Architects. Over his 45-year career, he designed more than 600 buildings, including banks, office buildings, churches, and synagogues. Nearly one-third of his commissions came from railroad companies. As chief architect of the Reading Railroad, he designed about 130 stations and industrial buildings. For the Pennsylvania Railroad, he designed more than 20 structures, including the great Broad Street Station (demolished 1953) at Broad and Market Streets in Philadelphia. His 40 stations for the Baltimore and Ohio Railroad included the ingenious 24th Street Station (demolished 1963) beside the Chestnut Street Bridge. His residential buildings included numerous mansions in Philadelphia and its suburbs, especially the Philadelphia Main Line and commissioned houses at the New Jersey Shore, and in Newport, Rhode Island, Bar Harbor, Maine, Washington, D.C.; New York state, and Chicago.

Furness broke from dogmatic adherence to European trends, and juxtaposed styles and elements in a forceful manner. His strong architectural will is seen in the unorthodox way he combined materials: stone, iron, glass, terra cotta, and brick. And his straightforward use of these materials, often in innovative or technologically advanced ways, reflected Philadelphia's industrial-realist culture of the post–Civil War period.

Furness designed custom furniture for a number of his early residences and buildings. One notable commission was the 1870–1871 redesign of the interiors of elder brother Horace Howard Furness's city house, at the southwest corner of 7th and Locust Streets in Philadelphia. Work on Horace's library included elaborate Neo-Grec bookcases, a reliquary for a (supposed) death mask of William Shakespeare, and a Neo-Grec desk, now at the Philadelphia Museum of Art. These pieces can be documented by drawings in Furness's sketchbooks and a letter in HHF's papers: "These bookcases were placed in position this day—February 18th 1871. They were designed by Capt. Frank Furness, and made by Daniel Pabst …"