Biomolecular condensates composed of proteins and RNA are one approach by which cells regulate post-transcriptional gene expression. Their formation typically involves the phase separation of intrinsically disordered proteins with a target mRNA, sequestering the mRNA into a liquid condensate. This sequestration regulates gene expression by modulating translation or facilitating RNA processing. Here we engineer synthetic condensates using a fusion of an RNA-binding protein, the human Pumilio2 homology domain (Pum2), and a synthetic intrinsically disordered protein, an elastin-like polypeptide (ELP), that can bind and sequester a target mRNA transcript. In protocells, sequestration of a target mRNA largely limits its translation. Conversely, in Escherichia coli, sequestration of the same target mRNA increases its translation. We characterize the Pum2-ELP condensate system using microscopy, biophysical and biochemical assays, and RNA sequencing. This approach enables the modulation of cell function via the formation of synthetic biomolecular condensates that regulate the expression of a target protein.