Land plants generally have a high capacity to generate new stem cells from differentiated cells, which permit de novo organ formation and vegetative propagation. The liverwort Marchantia polymorpha can asexually propagate using the specific organs called gemma cups and gemmae. We previously reported that the R2R3-MYB transcription factor (GCAM1), which is homologous to axillary meristem regulators in angiosperms (RAX/BLIND), are critical for gemma cup formation. To identify the target genes of GCAM1, we performed RNA-seq analyses using GCAM1-overexpressing plants. We found that a gene encoding a small protein with a single MYB domain is directly up-regulated by GCAM1 and named it GCAM1-Regulated One-R MYB (GROM). GROM expression was observed in apical notch, gemma cup, and sexual organs. Knock-out mutants of GROM produced downward-curling thalli without any gemma cups. Overexpression of GROM caused ectopic formation of gemma-like organs in wild-type (WT) background. Importantly, ectopic expression of GROM in gcam1 mutant partially rescued the defect of gemma cup formation, suggesting that GROM regulates gemma cup and gemma development as a direct target of GCAM1. While gcam1 shows no obvious defects other than gemma cup formation, grom mutants showed abnormal morphology of sexual organs as well as vegetative thallus. WT female crossed with male grom mutant produced normal sporangia, while female mutants crossed with male WT produced sporophytes arrested in immature stages. These results suggest that GROM also has GCAM1-independent functions in development of sexual reproductive phase.