LSD1 (KDM1A) is a histone demethylase that plays both oncogenic and tumor suppressor roles in breast cancer. However, the exact contexts under which it plays these opposite functions remain largely elusive. By characterizing its role in luminal breast epithelial cells, here we show that inhibition of LSD1 by both genetic and pharmacological approaches increases their invasion and migration, whereas its inhibition by genetic approach, but not by pharmacological approach, impairs their proliferation/survival. Induced loss of LSD1 in luminal cells in a mouse model of luminal breast cancer, MMTV-PyMT, leads to a profound increase in lung metastasis. Mechanistically, LSD1 interacts with GATA3, a key luminal-specific transcription factor (TF), and their common target genes are highly related to breast cancer. LSD1 positively regulates GATA3 expression. It also represses expression of TRIM37, a breast epithelial oncogene encoding a histone H2A ubiquitin ligase, and ELF5, a key TF gene for luminal progenitors and alveolar luminal cells. LSD1-loss also leads to reduced expression of several cell-cell adhesion genes (e.g., CDH1, VCL, CTNNA1), possibly via TRIM37-upregulation and subsequently TRIM37-mediated repression. Collectively, our data suggest LSD1 largely plays a tumor suppressor role in luminal breast cancer and the oncogenic program associated with LSD1-inhibition may be suppressed via TRIM37-inhibition.