The block lock for round link chains includes two approximately L-shaped lock halves connected with each other by pins. Each lock half has a longitudinal leg with a center rib, which faces away from its continuous back portion and separates bearing shells for chain links, and a cross leg arranged on one end of the longitudinal leg. Projections and receiving pockets are provided on the front ends of the longitudinal legs facing away from the cross legs, and receiving pockets and projections matching the projections and receiving pockets on the longitudinal legs are provided on the sides of the cross legs facing the center ribs. The center ribs are mutually offset in the longitudinal direction of the identically constructed lock halves.