Articulated pipe jack clamps include a pair of laterally opposed L-shaped legs hinged at an upper end to a central threaded collar, each having an elongate upper portion and a shorter inwardly turned bottom portion engageable with the underside of a wide V-shaped cradle of a pipe jack. A threaded stem engaged in the collar has a handle at its top end and an inverted V-shaped head connected at its bottom end by a ball-and-socket connection. The clamp is placed vertically downward over a work piece supported in the pipe jack cradle such that its V-shaped head is engaged on the top of the work piece, and the bottom portion of each leg is disposed beneath the sides of the cradle, then the head is screwed down to raise and engage the bottom portion of the legs on the underside of the cradle to clamp the work piece in the pipe jack.