Cyclic di-GMP (c-di-GMP) is a signalling molecule that governs the transition between planktonic and biofilm states. Previously, we showed that the diguanylate cyclase HmsT and the putative c-di-GMP phosphodiesterase HmsP inversely regulate biofilm formation through control of HmsHFRS-dependent poly-β-1,6-N-acetylglucosamine synthesis. Here, we systematically examine the functionality of the genes encoding putative c-di-GMP metabolic enzymes in Yersinia pestis. We determine that, in addition to hmsT and hmsP, only the gene y3730 encodes a functional enzyme capable of synthesizing c-di-GMP. The seven remaining genes are pseudogenes or encode proteins that do not function catalytically or are not expressed. Furthermore, we show that HmsP has c-di-GMP-specific phosphodiesterase activity. We report that a mutant incapable of c-di-GMP synthesis is unaffected in virulence in plague mouse models. Conversely, an hmsP mutant, unable to degrade c-di-GMP, is defective in virulence by a subcutaneous route of infection due to poly-β-1,6-N-acetylglucosamine overproduction. This suggests that c-di-GMP signalling is not only dispensable but deleterious for Y. pestis virulence. Our results show that a key event in the evolution of Y. pestis from the ancestral Yersinia pseudotuberculosis was a significant reduction in the complexity of its c-di-GMP signalling network likely resulting from the different disease cycles of these human pathogens.