We report a new p-type dopant, manganese oxide (Mn3O4) nanoparticle, to enhance the performance of organic field-effect transistors (OFETs) with conjugated polymers, including poly(3-hexylthiophene-2,5-diyl), poly[[N,N 9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)], and poly[[2,5-bis(2-octyldodecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[[2,2′-(2,5-thiophene)bis-thieno(3,2b) thiophene]-5,5′-diyl]] (DPPT-TT). Incorporating a small amount of Mn3O4 nanoparticles in the semiconductor film significantly improved the hole mobility and decreased the threshold voltage for all OFETs, indicating efficient Mn3O4 nanoparticle p-type doping. The Mn3O4 nanoparticle showed a better doping efficiency than the widely used FeCl3 dopant due to better mixability with the host conjugated polymers. In particular, doped DPPT-TT OFETs showed significantly improved mobility up to 2.35 (±0.4) cm2/(V·s) with enhanced air and operational stability at 0.1 wt % doping concentration from 1.2 cm2/(V·s) for pristine devices.