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Nuclear domains (NDs)—such as nucleoli or nuclear speckles—are membraneless, organelle-like compartments that concentrate and retain nuclear proteins. Despite their ubiquitous presence in the cell, the organization, regulation, and functions of many NDs remain poorly understood. The B-body is a prominent nuclear domain observed in developing flight muscles of Drosophila. In this study, we expand the understanding of B-body composition and function. We identify several additional RNA-binding proteins (RBPs) as B-body components and show that some proteins can dynamically disappear from this ND. We further demonstrate that the B-body contains an RNA component, which was identified as the long non-coding RNA hsrω. Genetic analyses reveal that hsrω acts as a structural scaffold for the B-body, and its depletion leads to B-body disassembly. In contrast, loss of the resident protein Bruno (Bru), a splicing factor, does not compromise B-body integrity. Finally, we show that imbalance in the hsrω/Bru ratio promotes Bru aggregation, suggesting that the B-body plays a role in maintaining nuclear protein homeostasis.