Abstract Purpose The folate receptor (FR) is frequently overexpressed in a variety of tumor types and, hence, an interesting target for radionuclide therapy. The aim of this study was to evaluate a new class of albumin-binding radioconjugates comprising 5-methyltetrahydrofolate (5-MTHF) as a targeting agent and to compare their properties with those of the previously established folic acid-based [¹⁷⁷Lu]Lu-OxFol-1. Methods [¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]Lu-6S-RedFol-1 were investigated in vitro using FR-positive KB tumor cells. Biodistribution studies were performed in KB tumor-bearing mice, and the areas under the curve (AUC_{0 → 120h}) were determined for the uptake in tumors and kidneys. [¹⁷⁷Lu]Lu-6R-RedFol-1 was compared with [¹⁷⁷Lu]Lu-OxFol-1 in a therapy study over 8 weeks using KB tumor-bearing mice. Results Both radioconjugates demonstrated similar in vitro properties as [¹⁷⁷Lu]Lu-OxFol-1; however, the tumor uptake of [¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]Lu-6S-RedFol-1 was significantly increased in comparison with [¹⁷⁷Lu]Lu-OxFol-1. In the case of [¹⁷⁷Lu]Lu-6S-RedFol-1, also the kidney uptake was increased; however, renal retention of [¹⁷⁷Lu]Lu-6R-RedFol-1 was similar to that of [¹⁷⁷Lu]Lu-OxFol-1. This led to an almost 4-fold increased tumor-to-kidney AUC_{0 → 120h} ratio of [¹⁷⁷Lu]Lu-6R-RedFol-1 as compared with [¹⁷⁷Lu]Lu-6S-RedFol-1 and [¹⁷⁷Lu]Lu-OxFol-1. At equal activity, the therapeutic effect of [¹⁷⁷Lu]Lu-6R-RedFol-1 was better than that of [¹⁷⁷Lu]Lu-OxFol-1, reflected by a slower tumor growth and, consequently, an increased median survival time (49 days vs. 34 days). Conclusion This study demonstrated the promising potential of 5-MTHF-based radioconjugates for FR-targeting. Application of [¹⁷⁷Lu]Lu-6R-RedFol-1 resulted in unprecedentedly high tumor-to-kidney ratios and, as a consequence, a superior therapeutic effect as compared with [¹⁷⁷Lu]Lu-OxFol-1. These findings, together with the absence of early side effects, make [¹⁷⁷Lu]Lu-6R-RedFol-1 attractive in view of a future clinical translation.