Gradius began development as a sequel to Konami's 1981 shoot 'em up Scramble under the name Scramble 2. According to director Hiroyasu Machiguchi, the decision to make it a horizontal shoot 'em up game was because they wanted to reuse as much of the materials for Scramble as possible during development.

Holding A+B while starting the game will display a hidden message: "KONAMI ONE GRADIUS ONE"

This appears to be a diagnostic screen, as some people have reported that it may say "TEN" instead of "ONE" on one line or the other. It is theorized that this screen displays the result of some kind of code verification algorithm or checksum, with each line representing the status of a different portion of the game's code.

A special version of Gradius was given away by ArchiMENdes, a ramen noodle company. The Vic Viper would power up using cups of ramen noodles rather than the usual pods.

While porting the game to the NES, one of the developers snuck in a code that gave you all the powerups, due to his inability to play the game. He then forgot to take the code out.

The code later became known as the Konami Code.