Abstract. The development of treatment resistance remains universal for patients with metastatic prostate cancer, driven by AR alterations and lineage state transitions. Identifying the evolution of lineage transitions in treatment resistance has been limited by the challenges of collecting serial tissue biopsies on treatment, which can be overcome using blood-based liquid biopsies. Utilizing a novel circulating tumor cell (CTC) isolation approach, we collected 273 CTC samples from 117 patients with metastatic prostate cancer for RNA sequencing. 146 samples from 70 patients had tumor purity comparable to tissue biopsies. We identified four CTC transcriptional phenotypes, mirroring lineage states identified in tissue. Patients with a luminal-B-like CTC phenotype defined by persistent AR signaling and high proliferation, as well as those with a neuroendocrine CTC phenotype, had significantly shorter survival than patients with luminal-A-like and low proliferation phenotypes. In a prospec