Tissue factor (TF) triggers blood coagulation and is translated from two mRNA splice isoforms, encoding membrane-anchored full-length TF (flTF) and soluble alternatively-spliced TF (asTF). The complete knockout of TF in mice causes embryonic lethality associated with failure of the yolk sac vasculature. Although asTF plays roles in postnatal angiogenesis, it is unknown whether it activates coagulation sufficiently or makes previously unrecognized contributions to sustaining integrity of embryonic yolk sac vessels. Using gene knock-in into the mouse TF locus, homozygous asTF knock-in (asTFKI) mice, which express murine asTF in the absence of flTF, exhibited embryonic lethality between day 9.5 and 10.5. Day 9.5 homozygous asTFKI embryos expressed asTF protein, but no procoagulant activity was detectable in a plasma clotting assay. Although the α-smooth-muscle-actin positive mesodermal layer as well as blood islands developed similarly in day 8.5 wild-type or homozygous asTFKI embryos, erythrocytes were progressively lost from disintegrating yolk sac vessels of asTFKI embryos by day 10.5. These data show that in the absence of flTF, asTF expressed during embryonic development has no measurable procoagulant activity, does not support embryonic vessel stability by non-coagulant mechanisms, and fails to maintain a functional vasculature and embryonic survival.