Recent studies have identified serotonylation of glutamine-5 on histone H3 (H3Q5ser) as a novel posttranslational modification (PTM) associated with active transcription. While H3Q5ser is known to be installed by tissue transglutaminase 2 (TGM2), the substrate characteristics affecting deposition of the mark, at the level of both chromatin and individual nucleosomes, remain poorly understood. Here, we show that histone serotonylation is excluded from constitutive heterochromatic regions in mammalian cells. Biochemical studies reveal that the formation of higher-order chromatin structures associated with heterochromatin impose a steric barrier that is refractory to TGM2-mediated histone monoaminylation. A series of structure-activity relationship studies, including the use of DNA-barcoded nucleosome libraries, shows that steric hindrance also steers TGM2 activity at the nucleosome level, restricting monoaminylation to accessible sites within histone tails. Collectively, our data indicate that the activity of TGM2 on chromatin is dictated by substrate accessibility rather than by primary sequence determinants or by the existence of preexisting PTMs, as is the case for many other histone-modifying enzymes.