We present a genome assembly from an individual maleThera britannica(the Spruce Carpet Moth; Arthropoda; Insecta; Lepidoptera; Geometridae). The genome sequence is 381 megabases in span. Most of the assembly is scaffolded into 19 chromosomal pseudomolecules, including the assembled Z sex chromosome. The mitochondrial genome has also been assembled and is 15.9 kilobases in length. Gene annotation of this assembly on Ensembl has identified 12,457 protein coding genes.