<p>Intermolecular [2+2] photocycloaddition reactions have emerged as a pivotal method for synthesizing four-membered cyclic compounds. However, no general, efficient protocol exists for accessing β-sultams or their analogues through intermolecular [2+2] photocycloaddition. Here we introduce an unconventional visible-light-mediated [2+2] photocycloaddition reaction between alkenes and sulfinylamines, facilitating the efficient synthesis of β-sultam derivatives that have garnered significant interest for their potential as covalent inhibitors in medicinal chemistry. Our approach features triplet energy transfer of sulfinylamines, enabling access to a broad range of β-sultams that were previously deemed inaccessible through conventional methods. Using a combination of experimental and computational techniques, we identify the origins of regio- and diastereo-selectivities, revealing the key factors influencing reaction efficiency. This method not only broadens the scope of available β-sultams and their derivatives but also highlights the potential of sulfinylamines as versatile reagents in organic synthesis.</p><p></p>