- For other uses, see J class starship.
A large design that rivaled the Constitution-class in size, it was considered old by the year 2266. Starfleet continued to use ships of the type for Starfleet Academy training missions, despite the feeling of some officers, such as Commodore José Mendez, that the class should be decommissioned. (TOS comic: "Captain's Log: Pike", TOS novel: Burning Dreams)
Although most Class J vessels had indeed been retired from service by the 2260s decade, Starfleet Command began a mass recommissioning of the class in 2267 as part of the effort to make Starfleet ready for the upcoming conflict with the Klingon Empire. (TOS novel: Demands of Honor)
Commodore Mendez identified and reported a number of structural flaws in the vessels, including the dangerously insufficient radiation safety systems in the engineering spaces. The engineering blast doors were too far down the connecting corridor to prevent radiation from spreading through the ships' Jefferies tubes and turbolift shafts. (TOS novel: Burning Dreams)