A passive self-adjusting seat assembly for attachment to a frame (14, 214) of a lawn maintenance vehicle, said passive self-adjusting seat assembly comprising: a base (24, 226) attached to said frame, said base including a pair of opposing side brackets connected by at least one cross bracket; a guide assembly connected to said base, said guide assembly including a front guide plate and a rear guide plate oriented parallel to each other and operatively connected to each other, wherein each of said guide plates includes a curved guide slot formed therein; a platform assembly including a seat support and a seat attached to said seat support, said platform assembly further including a plurality of guide wheels extending longitudinally forward or rearward from a front end of said seat support and longitudinally forward or rearward from a rear end of said seat support wherein each of said guide wheels is received within said guide slot of said front guide plate or said guide slot of said rear guide plate; and a locking assembly connected to said guide assembly for selectively engaging said platform assembly to prevent sliding or movement of said platform assembly relative to said guide assembly wherein said platform assembly is selectively slidable relative to said guide assembly in response to said lawn maintenance vehicle traversing uneven or sloped ground to maintain a user in a substantially vertical orientation, and said locking assembly includes a cam bolt attached to said front guide plate, an indexing rod at least partially positioned within said cam bolt, and a biasing member engages said cam bolt and said indexing rod for biasing said indexing rod longitudinally within said cam bolt. a base (24, 226) attached to said frame, said base including a pair of opposing side brackets connected by at least one cross bracket; a guide assembly connected to said base, said guide assembly including a front guide plate and a rear guide plate oriented parallel to each other and operatively connected to each other, wherein each of said guide plates includes a curved guide slot formed therein; a platform assembly including a seat support and a seat attached to said seat support, said platform assembly further including a plurality of guide wheels extending longitudinally forward or rearward from a front end of said seat support and longitudinally forward or rearward from a rear end of said seat support wherein each of said guide wheels is received within said guide slot of said front guide plate or said guide slot of said rear guide plate; and a locking assembly connected to said guide assembly for selectively engaging said platform assembly to prevent sliding or movement of said platform assembly relative to said guide assembly wherein said platform assembly is selectively slidable relative to said guide assembly in response to said lawn maintenance vehicle traversing uneven or sloped ground to maintain a user in a substantially vertical orientation, and said locking assembly includes a cam bolt attached to said front guide plate, an indexing rod at least partially positioned within said cam bolt, and a biasing member engages said cam bolt and said indexing rod for biasing said indexing rod longitudinally within said cam bolt.