A waterproof connector mateable with a mating connector along a front-rear direction, wherein: the waterproof connector comprises a terminal, a housing and a seal member; the terminal is held by the housing; the housing is provided with an attachment portion; when the attachment portion is viewed along the front-rear direction, the attachment portion has an outer circumferential shape including first sides and first coupling portions; the first coupling portions extend in directions which are different from one another; each of the first coupling portions couples the first sides, which are adjacent to each other, with each other; the seal member is formed of elastic material and is attached to the attachment portion; when the seal member is viewed along the front-rear direction, the seal member has a closed-ring shape including second sides and second coupling portions; the second sides correspond to the first sides, respectively; the second coupling portions extend in directions which are different from one another; each of the second coupling portions couples the second sides, which are adjacent to each other, with each other; the second sides include at least one specific second side; the at least one specific second side is provided with a front recess and a rear recess; the front recess is recessed rearward in the front-rear direction; the rear recess is recessed forward in the front-rear direction, characterized in that L 1 > L 2, where L 2 is a length of each of the second sides in a state before the seal member is attached to the attachment portion, and L 1 is a length of the corresponding first side; an expected amount G of distortion of each of the second sides upon attachment of the seal member to the attachment portion is defined by an equation of G = (L 1 - L 2) / L 2; the expected amount G of the distortion of the at least one specific second side is maximum among the expected amounts G of the distortions of the second sides; and a cross-section of a part of the at least one specific second side is smaller than a cross-section of any of the second sides other than the at least one specific second side, so that the at least one specific second side is easily stretched as compared to any of the second sides other than the specific second side. the waterproof connector comprises a terminal, a housing and a seal member; the terminal is held by the housing; the housing is provided with an attachment portion; when the attachment portion is viewed along the front-rear direction, the attachment portion has an outer circumferential shape including first sides and first coupling portions; the first coupling portions extend in directions which are different from one another; each of the first coupling portions couples the first sides, which are adjacent to each other, with each other; the seal member is formed of elastic material and is attached to the attachment portion; when the seal member is viewed along the front-rear direction, the seal member has a closed-ring shape including second sides and second coupling portions; the second sides correspond to the first sides, respectively; the second coupling portions extend in directions which are different from one another; each of the second coupling portions couples the second sides, which are adjacent to each other, with each other; the second sides include at least one specific second side; the at least one specific second side is provided with a front recess and a rear recess; the front recess is recessed rearward in the front-rear direction; the rear recess is recessed forward in the front-rear direction, characterized in that L 1 > L 2, where L 2 is a length of each of the second sides in a state before the seal member is attached to the attachment portion, and L 1 is a length of the corresponding first side; an expected amount G of distortion of each of the second sides upon attachment of the seal member to the attachment portion is defined by an equation of G = (L 1 - L 2) / L 2; the expected amount G of the distortion of the at least one specific second side is maximum among the expected amounts G of the distortions of the second sides; and a cross-section of a part of the at least one specific second side is smaller than a cross-section of any of the second sides other than the at least one specific second side, so that the at least one specific second side is easily stretched as compared to any of the second sides other than the specific second side.