We estimated the genome size of a relict longhorn beetle, Callipogon relictus Semenov (Cerambycidae: Prioninae)—the Korean natural monument no. 218 and a Class I endangered species—using a combination of flow cytometry and k-mer analysis. The two independent methods enabled accurate estimation of the genome size in Cerambycidae for the first time. The genome size of C. relictus was 1.8 ± 0.2 Gb, representing one of the largest cerambycid genomes studied to date. An accurate estimation of genome size of a critically endangered longhorned beetle is a major milestone in our understanding and characterization of the C. relictus genome. Ultimately, the findings provide useful insight into insect genomics and genome size evolution, particularly among beetles.