Recently, telomere length has been shown to be differentially regulated in asexually and sexually reproducing planarians. In addition, it was found that asexual worms maintain telomere length somatically during reproduction by fission or when regeneration is induced by amputation, whereas sexual worms only achieve telomere elongation through sexual reproduction. We have established an experimental bioassay system to induce switching from asexual to sexual reproduction in planarians, that is, sexualization. In this study, the relationship between the reproductive mode and telomere maintenance was investigated using innate asexually reproducing worms, innate sexually reproducing worms, and experimentally sexualized worms. Here, we show that innate asexual planarians maintain telomere length during cell division and that innate sexual planarians exhibit telomere shortening. However, experimental sexualized worms maintain telomere length during cell division. These results indicate that innate sexuality is linked to the mechanism of telomere maintenance.