Ribonuclease E (RNase E) has a key role in mRNA degradation and the processing of catalytic and structural RNAs in E. coli. We report the discovery of an evolutionarily conserved 17.4 kDa protein, here named RraA (regulator of ribonuclease activity A) that binds to RNase E and inhibits RNase E endonucleolytic cleavages without altering cleavage site specificity or interacting detectably with substrate RNAs. Overexpression of RraA circumvents the effects of an autoregulatory mechanism that normally maintains the RNase E cellular level within a narrow range, resulting in the genome-wide accumulation of RNase E-targeted transcripts. While not required for RraA action, the C-terminal RNase E region that serves as a scaffold for formation of a multiprotein degradosome complex modulates the inhibition of RNase E catalytic activity by RraA. Our results reveal a possible mechanism for the dynamic regulation of RNA decay and processing by inhibitory RNase binding proteins.