Antiphase boundaries (APB) in strontium ferromolybdate induce antiferromagnetically coupled spins of the Fe ions at the boundary affecting both the magnetic properties and the electron transport across the boundary. Herein, first, 3D presentation of the APB in strontium ferromolybdate is presented. For the first time, the weighted antiferromagnetic (AF) modulus, the bulk ferrimagnetic exchange stiffness constant, and the AF exchange stiffness constant across the APB of strontium ferromolybdate are estimated. This is done by analyzing carefully selected experimental data of the magnetoresistance (MR). The boundary width and boundary energy are calculated by means of Kittel's theory of the structure of ferromagnetic domains in films and small particles. The evaluation of these parameters deepens the understanding of MR in strontium ferromolybdate ceramic and thin‐film samples possessing a high density of APBs.