Background <p>The shear strength of an engineering material is a critical mechanical parameter, however, its measurement often encounters challenges especially for new materials. Moreover, little research was conducted on the size effect of the shear strengths.</p> Objective <p>This study is to determine the specimen thickness effect on the interlayer shear strengths of two types of additively manufactured polymers.</p> Methods <p>A combined experimental and numerical investigation of the interlayer shear strength measurement was conducted, and its application targeted polylactic acid and polyamide using fused filament fabrication and selective laser sintering, respectively. A necking-shaped shear specimen was proposed to measure the interlayer shear strengths with the aid of both 3D finite element analysis and 3D digital image correlation.</p> Results <p>All specimens showed a consistent pure shear fracture pattern, and the shear strengths increased as the specimen thickness increased.</p> Conclusions <p>Future interlayer shear strength measurements should specify a fixed specimen thickness for fair comparisons.</p>