Satisfactory treatment options for advanced leiomyosarcoma and liposarcoma are limited. The LEADER study (NCT03526679) investigated the safety and efficacy of lenvatinib plus eribulin. LEADER is a multicenter phase Ib/II study for advanced leiomyosarcoma or liposarcoma. The phase Ib part enrolled 6 patients to determine the dose-limiting toxicity (DLT) and recommended phase II dose (RP2D) with the starting dose of lenvatinib 18 mg/day and eribulin 1.1 mg/m2 D1, D8 every 21 days. The primary endpoint of the phase II part was objective response rate (ORR) based on Response Evaluation Criteria in Solid Tumors 1.1, with phase Ib patients preplanned to be included in the efficacy analysis. Translational analyses were based on the transcriptomic data obtained from the NanoString nCounter platform. Thirty patients were enrolled (leiomyosarcoma 21, liposarcoma 9); the median age was 59. One patient had to temporarily stop lenvatinib due to grade 2 arthritis in the first cycle, meeting DLT criteria. Four of 6 patients had to decrease the dose of lenvatinib to 14 mg between cycles two and three. RP2D was determined at lenvatinib 14 mg/day and eribulin 1.1 mg/m2. The confirmed ORR was 20%, and the ORR was not significantly different between phase Ib/II cohorts (P = 0.23). The median progression-free survival was 8.56 months (95% confidence interval, 4.40–not reached). Translational studies suggested increased dendritic cells in the tumor microenvironment (TME) after treatment. Lenvatinib plus eribulin has a manageable safety profile and exhibits promising efficacy for treating advanced leiomyosarcoma and liposarcoma.