T4 polynucleotide kinase is widely used for 5′-phosphorylation of DNA and RNA oligonucleotide termini, but no natural protein enzyme is capable of 3′-phosphorylation. Here, we report the in vitro selection of deoxyribozymes (DNA enzymes) capable of DNA oligonucleotide 3′-phosphorylation, using a 5′-triphosphorylated RNA transcript (pppRNA) as the phosphoryl donor. The basis of selection was the capture, during each selection round, of the 3′-phosphorylated DNA substrate terminus by 2-methylimidazole activation of the 3′-phosphate (forming 3′-MeImp) and subsequent splint ligation with a 5′-amino DNA oligonucleotide. Competing and precedented DNA-catalyzed reactions were DNA phosphodiester hydrolysis or deglycosylation, each also leading to a 3′-phosphate but at a different nucleotide position within the DNA substrate. One oligonucleotide 3′-kinase deoxyribozyme, obtained from an N40 random pool and named 3′Kin1, can 3′-phosphorylate nearly any DNA oligonucleotide substrate for which the 3′-terminus has the sequence motif 5′-NKR-3′, where N denotes any oligonucleotide sequence, K = T or G, and R = A or G. These results establish the viabilty of in vitro selection for identifying DNA enzymes that 3′-phosphorylate DNA oligonucleotides.