Despite a transition to 3D, the gameplay of Metal Gear Solid remains similar to its 2D MSX2 predecessor Metal Gear 2: Solid Snake. The player must navigate the protagonist, Solid Snake, through the game's areas without being detected by enemies. Detection is triggered by the player moving into an enemy's field of vision and sets off an alarm that draws armed enemies to his location. This also triggers "alert mode" and the player must then hide and remain undetected, at which point “evasion mode” begins and once the counter reaches zero the game returns to "infiltration mode" where enemies are not suspicious of Snake’s presence. The radar cannot be used in alert or evasion mode.
To remain undetected, the player can perform techniques which make use of both Solid Snake's abilities and the environment, such as crawling under objects, using boxes as cover, ducking or hiding around walls, and making noise to distract enemies. These are carried out using the third-person camera, which often changes its angle to give the player the best view of the area possible, and an on-screen radar, which displays enemies and their field of vision. Snake can also make use of many items and gadgets, such as infra-red goggles or a cardboard box disguise. The emphasis on stealth promotes a less violent form of gameplay, as fights against large groups of enemies will often result in serious damage for the player.
The game's progress is punctuated by cutscenes and codec, as well as encounters with bosses. To progress, players must discover the weaknesses of each boss and defeat them. Game controls and play strategies can also be accessed via the Codec radio, where advice is delivered from Snake's support team; for example, the support team may chastise Snake for not saving his progress often enough, or explain his combat moves in terms of which buttons to press on the gamepad. The Codec is also used to provide exposition on the game's backstory. Completion of the title provides the player with a statistical summary of their performance, and a "code name" based upon it, typically the name of a common animal.
In a first for the Metal Gear series, a training mode, called VR Mode, is available in which players can practice hiding techniques, weapon use, and sneaking. In addition to the stealth gameplay, there are set piece sequences that entail firefights between the player and enemies from the third-person and first-person perspectives.