The giant protein obscurin is thought to link the sarcomere with the sarcoplasmic reticulum (SR). The N-terminus of obscurin interacts with the M-band proteins titin and myomesin, whereas the C-terminus mediates interactions with ankyrin proteins. Here, we investigate the importance of obscurin for SR architecture and organization. Lack of obscurin in cross-striated muscles leads to changes in longitudinal SR architecture and disruption of small ankyrin-1.5 (sAnk1.5) expression and localization. Changes in SR architecture in obscurin knockout mice are also associated with alterations in several SR or SR-associated proteins, such as ankyrin-2 and β-spectrin. Finally, obscurin knockout mice display centralized nuclei in skeletal muscles as a sign of mild myopathy, but have normal sarcomeric structure and preserved muscle function.