In this study, linear friction welding is applied to join JIS-S45C medium carbon steel with ferrite and pearlite structures at temperatures above and below the A₁ point. Additionally, low-strain-rate tensile tests are conducted both in air and with a cathodic hydrogen charge to evaluate the hydrogen-embrittlement susceptibility of the linear friction-welded joints under both joining conditions. Results of hydrogen thermal-desorption analysis show that the hydrogen-charging conditions in this study simulated atmospheric corrosion conditions. The joining zone of the above-A₁ joint comprises fine martensite and ferrite, whereas that for the below-A₁ joint comprises ultrafine ferrite and cementite. In air tensile tests, both joints fractured in the base-metal region, thus suggesting the high reliability of the joints. In the hydrogen-charged tensile test, the above-A₁ joints exhibit premature fracture at the joining zone. By contrast, the below-A₁ joints exhibit base-metal fractures, thus suggesting that the joints are highly reliable in a hydrogen environment. Fracture-surface observations show that the above-A₁ joints exhibit cleavage fractures in the martensite-dominated region. Tensile tests on heat-treated martensite S45C specimens show that their fracture strength decreased significantly in a hydrogen environment. Therefore, the joint fracture is due to the significant decrease in the fracture strength of martensite formed in the above-A₁ joints in the hydrogen environment. The linear friction-welded medium carbon steel joints below the A₁ temperature can ensure reliability even in a hydrogen environment.