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The mechanisms enabling disseminating tumor cells to survive and colonize peritoneal niches during ovarian cancer metastasis remain incompletely understood. Here, we present MetTag, a single-cell barcoding and transcriptome profiling approach with time-stamped batch identifiers to resolve temporal dynamics of dissemination. Sequencing of MetTag barcodes reveals enrichment of early-disseminated clones across metastatic sites, and targeted depletion of pioneers diminishes the outgrowth of later arriving cells. MetTag-coupled single-cell RNA sequencing uncovers a distinct interferon-gamma-centric transcriptional trajectory enriched among pioneer clones. CRISPR/Cas9 screening and subsequent interferon-gamma receptor 1 knockout in pioneer cells significantly reduces metastatic burden, highlighting a critical window where active interferon-gamma signaling shapes the post-seeding metastatic niche and outgrowth. Mechanistically, the tumor-intrinsic interferon-gamma response and peritoneal macrophages cooperatively shield disseminating tumor cells from anoikis. Our study defines the temporal clonal architecture of peritoneal metastasis, revealing a first come, first served dissemination principle, where pioneer fitness determines the success of subsequent colonizers.