A novel functionalization of a ferromagnetic electrode employed in spintronic devices is reported. Self‐assembling monolayer technique has been used to chemisorb a paramagnetic phosphonate functionalized nitronyl‐nitroxide radical (NitPO) on the ferromagnetic La0.7Sr0.3MnO3 (LSMO) manganite surface. This interfacial layer causes clearly detectable modifications of the behavior in prototypical LSMO/NitPO/Gaq3/AlOx/Co vertical spintronic devices at temperatures below the ferromagnetic alignment (estimated by density functional theory) of the magnetic moments of NitPO and LSMO. This behavior can be justified by a significant spin filtering effect at the engineered interface, with a carrier selection (spin‐up) opposite to that of the LSMO/Gaq3 interface (spin‐down). It is proposed that the engineering of spin injecting interfaces with molecules having magnetic moment enables additional mechanisms to control …