The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.202201451.

Deoxyribonuclease‐I (DNase‐I), a representative endonuclease, is an important biomarker for the diagnosis of infectious diseases and cancer progression. However, enzymatic activity decreases rapidly ex vivo, which highlights the need for precise on‐site detection of DNase‐I. Here, a localized surface plasmon resonance (LSPR) biosensor that enables the simple and rapid detection of DNase‐I is reported. Moreover, a novel technique named electrochemical deposition and mild thermal annealing (EDMIT) is applied to overcome signal variations. By taking advantage of the low adhesion of gold clusters on indium tin oxide substrates, both the uniformity and sphericity of gold nanoparticles are increased under mild thermal annealing conditions via coalescence and Ostwald ripening. This ultimately results in an approximately 15‐fold decrease in LSPR signal variations. The linear range of the fabricated sensor is 20–1000 ng mL−1 with a limit of detection (LOD) of 127.25 pg mL−1, as demonstrated by spectral absorbance analyses. The fabricated LSPR sensor stably measured DNase‐I concentrations from samples collected from both an inflammatory bowel disease (IBD) mouse model, as well as human patients with severe COVID‐19 symptoms. Therefore, the proposed LSPR sensor fabricated via the EDMIT method can be used for early diagnosis of other infectious diseases.