核糖核苷酸还原酶 (英文:Ribonucleotide reductase.(RNR), 别称为 核糖核苷二磷酸还原酶) 他是一个酵素，功能为把核苷酸催化为脱氧核糖核苷酸. 脱氧核糖核苷酸常被使用于DNA的合成。而且在活体中物中，这个借由核糖核苷酸还原酶来催化的反应，受到非常严格的调控管理。 此外，核糖核苷酸还原酶在调节DNA的合成的总速率中扮演一个非常关键的角色，可以让我们在细胞分裂和DNA 复制时，DNA和细胞质量维持在固定比率。核糖核苷酸还原酶还有一个比较特别的调控方法，就是借由自由基机制来进行反应。核糖核苷酸还原酶的基质有 ADP, GDP, CDP and UDP. dTDP (脱氧胸苷二磷酸)是从dTMP (脱氧胸苷一磷酸)而来，借由(胸苷激酶)催化而形成.

Ribonucleotide reductase (RNR), also known as ribonucleoside diphosphate reductase, is an enzyme that catalyzes the formation of deoxyribonucleotides from ribonucleotides. Deoxyribonucleotides in turn are used in the synthesis of DNA. The reaction catalyzed by RNR is strictly conserved in all living organisms. Furthermore RNR plays a critical role in regulating the total rate of DNA synthesis so that DNA to cell mass is maintained at a constant ratio during cell division and DNA repair. A somewhat unusual feature of the RNR enzyme is that it catalyzes a reaction that proceeds via a free radical mechanism of action. The substrates for RNR are ADP, GDP, CDP and UDP. dTDP (deoxythymidine diphosphate) is synthesized by another enzyme (thymidylate kinase) from dTMP (deoxythymidine monophosphate).