The sarcomeric cytoskeleton is a network of modular proteins that integrate mechanical and signaling roles. Obscurin， or its homolog obscurin-like-1， bridges the giant ruler titin and the myosin crosslinker myomesin at the M-band. Yet， the molecular mechanisms underlying the physical obscurin(-like-1):myomesin connection， important for mechanical integrity of the M-band， remained elusive. Here， using a combination of structural， cellular， and single-molecule force spectroscopy techniques， we decode the architectural and functional determinants defining the obscurin(-like-1):myomesin complex. The crystal structure reveals a trans-complementation mechanism whereby an incomplete immunoglobulin-like domain assimilates an isoform-specific myomesin interdomain sequence. Crucially， this unconventional architecture provides mechanical stability up to forces of ∼135 pN. A cellular competition assay in neonatal rat cardiomyocytes validates the complex and provides the rationale for the isoform specificity of the interaction. Altogether， our results reveal a novel binding strategy in sarcomere assembly， which might have implications on muscle nanomechanics and overall M-band organization.