<p>Accurate prenatal diagnosis of high-risk placenta accreta spectrum (PAS) is vital for maternal safety and can mitigate severe complications. We aimed to investigate the diagnostic value of a novel MRI sign, “increased intra-myometrial vascularity (IIMV),” and to evaluate its contribution to an optimal MRI sign combination for differentiating high-risk placenta accreta spectrum (PAS), specifically placenta increta (PI) and percreta (PP), from placenta accreta (PA) or normal placenta. This retrospective study reviewed 166 high-risk pregnant women who underwent MRI. Two radiologists independently assessed the presence of the seven established risk signs from the SAR-ESUR consensus and the novel IIMV sign. The IIMV sign was defined as continuous tubular or tortuous flow-void structures confined within the myometrium, associated with architectural distortion. Univariate logistic regression and interobserver agreement (kappa statistics) were used to select significant signs, which were then incorporated into a multivariate model to construct diagnostic combinations. AUCs were compared between combinations (with and without IIMV sign) in differentiating PI + PP group from PA + Normal groups. The novel IIMV sign demonstrated the highest specificity (0.91) and was independently associated with the differentiation between the PI + PP and Normal + PA groups. The optimal sign combination, which included IIMV, placental bulge, myometrial thinning, and bladder wall interruption, achieved an area under the curve (AUC) of 0.84. In contrast, the combination without IIMV showed a decreased AUC of 0.79 (<i>p</i> = 0.1203). The “increased intra-myometrial vascularity” sign is a highly specific marker for high-risk PAS. Its inclusion in an MRI-based diagnostic model improves the performance for identifying invasive placental disorders compared to combinations that do not utilize this novel sign.</p>