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In this study, in situ formed silica nanoparticles (SNPs) emitting second-level phosphorescence at room temperature without a phosphorescent dopant have been achieved for the first time. This phosphorescence is achieved through the simple in situ formation of carbonaceous defects (CDs) within the SNPs, followed by passivation of the CDs by a robust silica matrix. The CD in the SNPs, termed CD@SNPs, are synthesized by cross-linking tetraethyl orthosilicate (TEOS) and (3-aminopropyl)triethoxysilane (APTES), and these cross-linked components create a porous structure within the silica matrix. Upon calcination, the carbon-related structures within the pores deform, leading to the formation of CDs. Confined within a robust silica matrix, the molecular motion of the CD is restricted, facilitating the generation of a stable triplet state and suppressing nonradiative decay. Moreover, the robust silica matrix passivates the CD confined in the SNPs at a nanoscale. These comprehensive effects enable prolonged phosphorescence emission from the SNPs. In addition, these phosphorescence-emitting SNPs have been applied as dopants in the emissive layer of organic light-emitting diodes to realize blue light emission. This feature suggests the possibility of utilizing luminescent SNPs as light emitters in display technologies.