A chimeric RNase A inhibitor (SUMO-RI) was produced by fusing a SUMO domain to the N-terminus of the murine Rnh1 protein. Functional assays demonstrated that SUMO-RI effectively protects RNA from RNase A-mediated degradation under conditions mimicking real-time RT-PCR, with performance comparable to that of commercial RNase inhibitors. The primary advantage of the chimeric design is its improved technological suitability: SUMO-RI exhibits markedly enhanced storage stability relative to the recombinant Rnh1 inhibitor. However, this benefit comes with a trade-off—SUMO fusion reduces thermostability at temperatures above approximately 47 °C. Together, these findings establish SUMO fusion as a rational engineering strategy for RNase inhibitors, offering improved practical handling at the expense of thermal resilience.