A keystone jack assembly includes a jack housing, a piercing contact housing disposed on the jack housing, a plurality of piercing contacts mounted on the piercing contact housing, a wire cap movably disposed on the piercing contact housing and a cover pivotally connected to the jack housing. The wire cap includes a cap main body, at least one first guiding portion, at least one second guiding portion and a third guiding portion. The cover includes two covering parts, and each covering part includes a main body, at least one first contact portion, at least one second contact portion and a third contact portion. The first contact portion abuts against the first guiding portion, the second contact portion abuts against the second guiding portion and the third contact portion abuts against the third guiding portion in order as the two covering parts pivots toward each other.