Frederick Barnard Fetherstonhaugh KC (June 2, 1863 – July 6, 1945) was a Canadian patent lawyer and agent who founded the patent agency Fetherstonhaugh & Co. He was widely recognized for his expertise in patent law. Fetherstonhaugh was also the owner of one of the first electric cars in Canada.

Frederick Bernard Fetherstonhaugh was born in Paisley, Ontario on June 2, 1863, to Francis Fetherstonhaugh and Frances Swift. He was a descendant of the Irish Fetherstonhaughs of Carrick House in County Westmeath. His father immigrated to Canada around 1859 and resided in various towns in Ontario including Orillia, Guelph, and Paisley, until he settled in Toronto with his family.

Fetherstonhaugh attended the Ryerson School in his youth, where he excelled academically, eventually winning a scholarship to the Toronto Collegiate Institute for two years. He then attended the University of Toronto, taking courses at the School of Practical Science in Mechanical Engineering. Before finishing his diploma, however, he decided to pursue the study of law.

He married Marion Arabella Rutledge in February 1887 and had four children, though only the eldest—James Edward Marion (b. 1888)—survived into adulthood. Godfrey Orme (b. 1889) died of a cerebral hemorrhage after his nurse lost control of his baby carriage; Marion Vivian (b. 1890) died of "infantile convulsions" in 1892; and Frederic Harold (b. 1892) died on the eve of his fifth birthday when he was struck by a streetcar while crossing Spadina Ave in front of the family home near Harbord St. His eldest son, James Edward Marion, would later attain the rank of major in the Canadian Army after serving in World War I and joined his father's patent business after the war as an engineering expert.

Fetherstonhaugh began his career with the patent law firm Donald C. Ridout & Co, where he was a draftsman along with Charles Riches and James E. Maybee before his call to the bar in 1889. One year after his bar admission, in 1890, he formed his own firm, Fetherstonhaugh & Co., where he took up practice as a patent lawyer.

The firm Fetherstonhaugh & Co. opened its first location on the second floor of the Bank of Commerce Building at 19-25 King Street W, and was the first tenant of the building. His patent law practice expanded quickly. In 1895, an Ottawa office was opened at 7 Elgin Street, managed by T.S. Young. In 1896, he opened a Montreal office with his brother, Edward James Fetherstonhaugh. By 1905, offices had been opened in New York City (managed by Albert T. Nathan, a former patent examiner) and in Washington, D.C.

In 1904, Russel S. Smart joined his firm as the manager of the Ottawa office. Smart later took over the legal practice of the firm while Fetherstonhaugh continued his patent agency, forming a partnership that has continued to this day as Smart & Biggar/Fetherstonhaugh. By the time of Fetherstonhaugh's death in 1945, several more offices had been established across Canada, including in Hamilton, Vancouver, Winnipeg, Halifax, and Saint John.

During his career, Fetherstonhaugh was recognized as a leading expert in his field and was regularly consulted by Parliament regarding legislative amendments to patent laws and regulations. Around 1905, he advocated for uniformity of patent legislation within the British Empire, arguing that it would enhance scientific progress and reduce costs for inventors obtaining patents in multiple jurisdictions. In support of this proposal, Fetherstonhaugh drafted the Empire Patent Act, which was designed to coordinate the policies of patent offices across the British Empire. However, his proposed unification of patent laws in the British Empire never came to pass. Fetherstonhaugh was made a King's Counsel in 1910.