Active STAT3 pathway promotes epithelial mesenchymal transition, migration, invasion and metastasis in cancer. STAT3 upregulates the transcription of epithelial mesenchymal transition key transcription factor SNAIL through DNA-binding independent way. However, the mechanism that how is STAT3 recruited to SNAIL promoter to upregulate transcription in DNA independent way is still unclear. In our study, we found a lysine methylated binding protein L3MBTL3 was positively associated with metastatic and poor prognosis in breast cancer. L3MBTL3 also promoted epithelial-mesenchymal transition, migration, invasion and metastasis in breast cancer. Mechanism analysis found L3MBTL3 interacted with active STAT3 and recruited active STAT3 on SNAIL promoter to up-regulated SNAIL transcription. The interaction between L3MBTL3 and active STAT3 was required for SNAIL transcription up-regulation, migration and invasion in breast cancer, while the methylated lysine binding activity of L3MBTL3 is not required for these functions. In conclusion, L3MBTL3 and active STAT3 collaboratively up-regulate SNAIL transcription to promote breast cancer metastasis.