The moss Physcomitrella patens is an important model organism for evo‐devo studies. Here, we determined the genome‐wide chromatin landscape of five important histone three (H3) modifications ( H 3 K 4me3, H 3 K 27me3, H 3 K 27 A c, H 3 K 9 A c and H 3 K 9me2) and describe the changes to these histone marks in two contrasted situations, developmental transition and abiotic (drought) stress. Integrative analysis of these histone H 3 modifications revealed their preferential association into 15 chromatin states ( CS ) in genic regions of the P. patens genome. Synergistic relationships that influence expression levels were revealed for the three activating marks H 3 K 4me3, H 3 K 27 A c and H 3 K 9 A c, while an antagonistic relationship was found between CS containing the H 3 K 27me3 and H 3 K 27 A c marks, suggesting that H 3 K 27 is a key indexing residue regarding transcriptional output. Concerning the alteration of histone marks in response to developmental transition (juvenile to adult) and drought stress, the three activating marks H 3 K 4me3, H 3 K 27 A c and H 3 K 9 A c show significant changes in both situations. However, changes to H 3 K 27me3 are central only for genes differentially expressed during development. Interestingly, genes induced during drought stress show significant histone mark toggling during developmental transition. This situation suggests that drought induced adult (gametophore expressed) genes are primed to respond to this stress during the juvenile to adult transition.