The cell nucleus is a highly compartmentalized organelle harbouring a variety of dynamic membraneless nuclear bodies. How these subnuclear domains are established and maintained is not well understood. Here, we investigate the molecular mechanism of how one nuclear body, the paraspeckle, is assembled and organized. Paraspeckles are discrete ribonucleoprotein bodies found in mammalian cells and implicated in nuclear retention of hyperedited mRNAs. We developed a live-cell imaging system that allows for the inducible transcription of Men /β (also known as Neat1; ref. 12) noncoding RNAs (ncRNAs) and the direct visualization of the recruitment of paraspeckle proteins. Using this system, we demonstrate that Men /β ncRNAs are essential to initiate the de novo assembly of paraspeckles. These newly formed structures effectively harbour nuclear-retained mRNAs confirming that they are bona fide functional paraspeckles. By three independent approaches, we show that it is the act of Men /β transcription, but not ncRNAs alone, that regulates paraspeckle maintenance. Finally, fluorescence recovery after photobleaching (FRAP) analyses supported a critical structural role for Men /β ncRNAs in paraspeckle organization. This study establishes a model in which Men /β ncRNAs serve as a platform to recruit proteins to assemble paraspeckles.