A drive force transmission device includes a motor, a transmission, a motor case, and a seal member. The motor is disposed in a dry space within a motor housing. The transmission is disposed in a wet space within a transmission case. The motor case covers an outer periphery of the motor housing and is coupled to the transmission case. The seal member is between the motor case and the motor housing and isolates the wet space and the outer space. The motor housing is divided into a divider wall housing dividing off the wet space and an outer wall housing dividing off the outer space, the divider wall housing being a material lighter than a material of the outer wall housing and integrally coupled thereto. The divider wall housing is divided from the outer wall housing at a position nearer to the dry space than the seal member.