jats:pArchaeal fibrillarin (aFib) is a well-characterized jats:italicS</jats:italic>-adenosyl methionine (SAM)-dependent RNA 2′-jats:italicO</jats:italic>-methyltransferase that is known to act in a large C/D ribonucleoprotein (RNP) complex together with Nop5 and L7Ae proteins and a box C/D guide RNA. In the reaction, the guide RNA serves to direct the methylation reaction to a specific site in tRNA or rRNA by sequence complementarity. Here we show that a jats:italicPyrococcus abyssi</jats:italic> aFib–Nop5 heterodimer can alone perform SAM-dependent 2′-jats:italicO</jats:italic>-methylation of 16S and 23S ribosomal RNAs in vitro independently of L7Ae and C/D guide RNAs. Using tritium-labeling, mass spectrometry, and reverse transcription analysis, we identified three in vitro 2′-jats:italicO</jats:italic>-methylated positions in the 16S rRNA of jats:italicP. abyssi</jats:italic>, positions lying outside of previously reported pyrococcal C/D RNP methylation sites. This newly discovered stand-alone activity of aFib–Nop5 may provide an example of an ancestral activity retained in enzymes that were recruited to larger complexes during evolution.</jats:p>