The Girl on a Motorcycle (French: La Motocyclette) is a 1968 erotic romantic drama film directed by Jack Cardiff, starring Alain Delon and Marianne Faithfull. It is based on the 1963 novel La Motocyclette by André Pieyre de Mandiargues. Released as Naked Under Leather, it was the first film to receive an X rating in the United States, and edited by Warner Brothers for an "R" rating. It was listed to compete at the 1968 Cannes Film Festival, but the festival was cancelled due to the May 1968 events in France.

The Girl on a Motorcycle redefined the leather jacket for motorcyclists into a catsuit, created by John Sutcliffe, that Faithfull wore in the film.

The film is set in France and Germany. Newly married Rebecca Nul leaves her husband Raymond's bed on her prized motorbike—her symbol of freedom and escape. During her ride to visit Daniel Lionart, her lover in Heidelberg, she indulges in psychedelic and erotic reveries as she relives her changing relationship with the two men.

Flashbacks reveal that Rebecca met Daniel while working at her father's bookshop a few weeks before her marriage to Raymond, a school teacher. Daniel gives Rebecca motorcycle driving lessons on a Norton Atlas motorcycle. They quickly become lovers. She asks him if he will marry her, and he simply says "No". She tells him that she is soon to be married, and he says he will give her a wedding present. A Harley Davidson Electra Glide motorcycle is delivered to Rebecca at the bookshop. Her father says she should refuse it, but she asks Raymond what she should do and he says she should do what will make her happy.

Rebecca rides the motorcycle from France to Germany to be with Daniel. After having several drinks of kirsch in a small village bar, she drives fast and recklessly, and although she wears leathers she has no helmet, and her ride to Daniel ends prematurely when she collides with a truck which swerves in front of her, throwing her head-first through the windscreen of an oncoming car, which then crashes into the truck.

The film was made on locations in France (including Haguenau), Germany (including Neulauterburg), Switzerland (including Geneva), and Belgium. The bookshop scenes were filmed at the A. Jullien bookshop in Bourg-de-Four Geneva. In many of the scenes where Marianne Faithfull is seen driving the Harley Davidson Electra Glide motorcycle at high speeds, she was seated on the motorcycle which was attached to a platform that was being pulled by a vehicle in front of it. Many of the scenes were filmed in the English language and then again in the French language.

Jack Cardiff received extensive cooperation from the police in blocking the roads, so many scenes show only the lone motorcycle and no other vehicular traffic. The final crash scene was staged on an abandoned airstrip in England. The psychedelic colour effects were achieved by solarizing the film during post-production. Faithfull's riding double in medium to distant shots was Bill Ivy, British Grand Prix motorcycle champion.

The Girl on a Motorcycle was the sixth most popular film in general release in Britain during 1968.