This article argues that the short textual fragment today called the Fifth Grammatical Treatise (below 5GT) represents a kind of hybrid within Icelandic poetics, occupying a middle ground between the nativizing treatises, such as Snorri’s Edda, and the Latinate ones, such as 3GT and 4GT. Like Snorri’s Edda, 5GT uses vernacular rather than Latin terminology and it is not arranged according to principles canonized within the grammatical tradition, but the concept it treats are alien to the vernacular tradition and belong in the sphere of Latin learning. The article also contends that the combined evidence of manuscript context and features within the text itself suggest that its author is Óláfr Þórðarson, nephew of Snorri Sturluson, or someone very close to him, and that Óláfr drew on 5GT in his composition of 3GT. This contextualization sheds further light on how 5GT fits into the evolving tradition of Icelandic poetics, not least because Óláfr is the person who took this tradition from a nativizing to a Latinate mode in 3GT. The attribution of 5GT to his intellectual milieu thus enables us to get close to the individuals who, after Snorri, developed the discourse on vernacular poetics in the thirteenth century. The translation of Latin texts that Óláfr undertook in 3GT may appear as the most straightforward way of transferring Latin learning into the vernacular. The intermediate, hybrid solution of 5GT, however, suggests that the nativizing mode held such a strong position in Icelandic intellectual circles that translation was the most, rather than the least, challenging solution for transforming local poetics into a theoretical discourse recognizable by European standards.