We present a genome assembly of Polytrichum formosum (Bank Haircap; Streptophyta; Polytrichopsida; Polytrichales; Polytrichaceae). The genome sequence has a total length of 487.54 megabases. Most of the assembly (97.99%) is scaffolded into 7 chromosomal pseudomolecules. The mitochondrial sequence has a length of 114.81 kilobases and the plastid genome assembly has a length of 126.24 kilobases. Gene annotation of this assembly on Ensembl identified 16 773 protein-coding genes. This assembly was generated as part of the Darwin Tree of Life project, which produces reference genomes for eukaryotic species found in Britain and Ireland.