Recently, artificial ribonucleases (aRNases)—conjugates of oligodeoxyribonucleotides and peptide (LR)4-G-amide—were designed and assessed in terms of the activity and specificity of RNA cleavage. The conjugates were shown to cleave RNA at Pyr-A and G–X sequences. Variations of oligonucleotide length and sequence, peptide and linker structure led to the development of conjugates exhibiting G–X cleavage specificity only. The most efficient catalyst is built of nonadeoxyribonucleotide of unique sequence and peptide (LR)4-G-NH2 connected by the linker of three abasic deoxyribonucleotides (conjugate pep-9). Investigation of the cleavage specificity of conjugate pep-9 showed that the compound is the first single-stranded guanine-specific aRNase, which mimics RNase T1. Rate enhancement of RNA cleavage at G–X linkages catalysed by pep-9 is 108 compared to non-catalysed reaction, pep-9 cleaves these linkages only 105-fold less efficiently than RNase T1 (kcat_RNase T1/kcat_pep-9 = 105).