We present a genome assembly of Arenaria serpyllifolia (Thyme-leaved Sandwort; Streptophyta; Magnoliopsida; Caryophyllales; Caryophyllaceae). The assembly consists of two haplotypes with total lengths of 726.25 megabases and 713.02 megabases. Most of haplotype 1 (97.75%) is scaffolded into 20 chromosomal pseudomolecules. Haplotype 2 was assembled to scaffold level. The mitochondrial sequences have lengths of 80.46, 32.01, 32.06, 29.14 and 32.88 kilobases and the plastid genome assembly has a length of 146.66 kilobases. Gene annotation of this assembly on Ensembl identified 46 083 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.