Stellaris is a real-time grand strategy game set in space, beginning in the year 2200. Players control ships, including science, construction and military vessels. Combat is more centered towards the bigger picture, preparation, and strategy. There are also diplomatic options such as alliances and trade agreements with other races.The game begins by picking a species, its ethics, and its technologies. Picking an alien and changing their ethics can change the way information looks; and any species can have any of the ship engineering styles.

The player begins with a single planet in their territory, a construction ship, a science ship, three small warships, and a spaceport. Early gameplay consists of exploring and colonizing space, while mid-game activities focus on diplomacy and governing an empire. In the late game, crisis events can occur that have galaxy-wide implications—for example, an uprising by sentient AI robots or an invasion by extra-dimensional or extra-galactic forces, the former two always being triggered by careless empires. Paradox hoped that this would address a common late-game problem in 4X style games; whereby one faction is so powerful that their eventual victory is inevitable, resulting in frustrating gameplay