A metal‐free photosensitized protocol for regioselective diamination of alkene feedstocks over a single step was developed based on the rationally designed bifunctional diamination reagent, thus affording a range of differentially protected 1,2‐diamines in moderate to high yields. Mechanistic studies reveal that the reaction is initiated with a triplet‐triplet energy transfer between thioxanthone catalyst and diamination reagent, followed by fragmentation to simultaneously generate long‐lived iminyl radical and transient amidyl radical. The excellent regioselectivity presumably stems from the large reactivity difference between two different N‐centered radical species. This protocol is characterized by excellent regioselectivity, broad functional group tolerance, and mild reaction conditions, which would enrich the diversity and versatility of facilitate the diversity‐oriented synthesis of 1,2‐diamine‐containing complex molecule scaffolds. A metal‐free, photosensitized regioselective diamination of alkene is reported. The keys to this advance are: 1) rational design of bifunctional diamination reagent, 2) simultaneous generation of long‐lived iminyl radical and transient amidyl radical with significantly differed reactivities, and 3) involvement of photoinduced energy transfer reaction mode.