The human cytomegalovirus (HCMV) gene UL21a was recently annotated by its conservation in chimpanzee cytomegalovirus. Two large-scale mutagenic analyses showed that mutations in overlapping UL21a/UL21 resulted in a severe defect of virus growth in fibroblasts. Here, we characterized UL21a and demonstrated its role in HCMV infection. We mapped a UL21a-specific transcript of approximately 600 bp that was expressed with early kinetics. UL21a encoded pUL21a, a protein of approximately 15 kDa, which was unstable and localized predominantly to the cytoplasm during HCMV infection or when expressed alone. Interestingly, pUL21a was drastically stabilized in the presence of proteasome inhibitor MG132, but its instability was independent of a functional ubiquitin-mediated pathway, suggesting that pUL21a underwent proteasome-dependent, ubiquitin-independent degradation. A UL21a deletion virus was attenuated in primary human newborn foreskin fibroblasts (HFFs) and embryonic lung fibroblasts (MRC-5), whereas a marker-rescued virus and mutant viruses lacking the neighboring or overlapping genes UL20, UL21, or UL21.5-UL23 replicated at wild-type levels. The growth defect of UL21a-deficient virus in MRC-5 cells was more pronounced than that in HFFs. At a high multiplicity of infection, the UL21a deletion virus synthesized viral proteins with wild-type kinetics but had a two- to threefold defect in viral DNA replication. More importantly, although pUL21a was not detected in the virion, progeny virions produced by the mutant virus were approximately 10 times less infectious than wild-type virus, suggesting that UL21a is required for HCMV to establish efficient productive infection. We conclude that UL21a encodes a short-lived cytoplasmic protein and facilitates HCMV replication in fibroblasts.