An efficient asymmetric 1,3‐dipolar cycloaddition reaction between α‐substituted diazophosphonates and 3‐acryloyl‐2‐oxazolidinone, catalyzed by a chiral Mg(II)/BOX complex, has been achieved for the first time. This reaction gave the corresponding chiral 5,5‐disubstituted 1H‐pyrazoline‐5‐phosphonates in good yields (up to 98%) and excellent stereoselectivities (up to 95% ee). The resulting highly functionalized pyrazolines, which have a tetrasubstituted stereogenic center at the phosphonate‐bearing carbon, can be further transformed into pyrazolidine and bicyclic pyrazolidine derivatives.magnified image