J class or Class J may refer to:

• NZR J class (1874), steam locomotives operating in New Zealand from 1874 until 1935
• NZR J class (1939), steam locomotives operating in New Zealand from 1939
• WAGR J class, steam locomotives operating in Western Australia from the 1890s
• WAGR J class (diesel), diesel locomotives operating in Western Australia from the 1960s
• Victorian Railways J class (1859), main line steam locomotives operating in Victoria, Australia from 1859 to 1912
• Victorian Railways J class (1954), branch line steam locomotives operating in Victoria, Australia from 1954 to 1972
• Norfolk and Western Railway class J (1879), steam locomotives operating in the United States from 1879 until 1900
• Norfolk and Western Railway class J (1903), steam locomotives operating in the United States from 1903 until 1935
• Norfolk and Western Railway class J (1941), steam locomotives operating in the United States from 1950 until 1959
• J-class Melbourne tram
• J-class Sydney tram

• J-class yacht, a racing yacht design constructed for the America's Cup competition between 1931 and 1937
• J-class destroyer, a class of British-built destroyers active from 1937 until 1949
• J-class submarine, a class of British-built submarines operated by the Royal Navy and Royal Australian Navy between 1916 and 1930
• Jymy-class motor torpedo boat, an Italian World War II boat

• J-class blimp, 1920s airships
• J-segment, a European vehicle size class
• Class. J., the ISO 4 abbreviation for The Classical Journal.
• A code used by many airlines for business class

• J type (disambiguation)