Pachycerianthus multiplicatus is a tube-dwelling ceriantharian anemone (family Cerianthidae) found in subtidal soft sediments of the north-east Atlantic and the Mediterranean, where it lives in a parchment-like tube that it secretes itself. Ceriantharia is the earliest-diverging lineage of Anthozoa and a key outgroup for cnidarian phylogenetics, which makes this genome a useful addition to the site. The assembly registered here (GCA_984695265.1, 602.6 Mb, 18 chromosomes) was produced by Genoscope as part of the ATLASea programme from PacBio long reads and Arima Hi-C data. It is the alternate haplotype of jaPacMult1; the companion primary assembly is GCA_984588385.1 (672.2 Mb, 19 chromosomes). No gene annotation has been released for either assembly, so this species is currently registered at genome level only: gene models and functional annotation will be added once an annotation becomes available. BUSCO completeness has nonetheless been assessed directly on the assembly, in genome mode with miniprot as the gene predictor: C:92.8% [S:91.8%, D:1.0%], F:2.8%, M:4.4% (n = 3,203; lineage cnidaria_odb12). Two caveats apply to this figure. It comes from genes predicted during the assessment rather than from a deposited protein set, which is how the other species on this site were scored, so it is not strictly comparable with them; and 30.1% of the complete matches contain internal stop codons, a known artefact of miniprot prediction that inflates the completeness estimate somewhat. Note also that taxonomic authorities differ on the placement of Ceriantharia — WoRMS (AphiaID 101013) treats it as an order within Hexacorallia, whereas NCBI (taxid 1531352) places it alongside Hexacorallia and Octocorallia; the site uses the latter treatment here.