Silver compounds have favorable properties as promising anticancer drug candidates, such as low side effects, anti-inflammatory properties, and high potential to overcome drug resistance. However, the exact mechanism by which Ag(I) confers anticancer activity remains unclear, which hinders further development of anticancer applications of silver compounds. Here, we combine thermal proteome profiling, cysteine profiling, and ubiquitome profiling to study the molecular mechanisms of silver(I) complexes supported by non-toxic thiourea (TU) ligands. Through the formation of AgTU complexes, TU ligands deliver Ag⁺ ions to cancer cells and tumour xenografts to elicit inhibitory potency. Our chemical proteomics studies show that AgTU acts on the ubiquitin-proteasome system (UPS) and disrupts protein homeostasis, which has been identified as a main anticancer mechanism. Specifically, Ag⁺ ions are released from AgTU in the cellular environment, directly target the 19S proteasome regulatory complex, and may oxidize its cysteine residues, thereby inhibiting proteasomal activity and accumulating ubiquitinated proteins. After AgTU treatment, proteasome subunits are massively ubiquitinated and aberrantly aggregated, leading to impaired protein homeostasis and paraptotic death of cancer cells. This work reveals the unique anticancer mechanism of Ag(I) targeting the 19S proteasome regulatory complex and opens up new avenues for optimizing silver-based anticancer efficacy.