The title Sonatine comes from the musical term sonatina. Kitano said that when learning the piano, when the learner gets to sonatinas they have to decide where they want to go, whether it is to classical, jazz or popular music; marking the point of crucial decision making. This refers to the character Murakawa in the film.
The film's poster is of a Napoleon fish being pierced with a spear. Kitano said this type of fish used to be very common in the oceans south of Japan, but has been decreasing. He used the image simply because the contrast of the 'beautifully shaped' fish being speared seemed striking to him.