An electrical connector assembly comprises a first connector including a first housing having a mating opening, a second connector including a second housing configured to be mated with the mating opening of the first connector, and a drainage structure provided on and corresponding to a boundary portion between the first housing and the second housing. The boundary portion faces the mating opening. The drainage structure includes a convex region having a top at the boundary portion, a first concave region continuous with the convex region and formed in the first housing, and a second concave region continuous with the convex region and formed in the second housing.