The structures of four new isoatisine-type alkaloids, spiramines A, B, C, and D, isolated from Spiraea japonica L. fil var acuminata Franch, were determined. Spiramines A and B are epimeric at C-19, as are spiramines C and D. The basic skeleton of spiramine A (C24H33NO4, mp 137.5–139°C, [α]²⁵ᴰ -103.1°) was established via 13C NMR comparison with literature data. Hydrolysis of spiramine A yielded spiramines C (C22H31NO3, mp 167–169°C, [α]²⁵ᴰ -149.9°) and D (C22H31NO3, mp 160–162°C, [α]²⁵ᴰ -169.0°), which are C-19 epimers. Spiramine A’s structure—comprising an isoatisine-type oxazolidine ring, an ether linkage between C-7 and C-20, a C-15 secondary acetoxy group, and an exo methylene group at C-16/C-17—was elucidated using 1H/13C NMR, irradiation experiments (confirming the acetoxy group’s location), reduction (to triol 7), and oxidation/reduction of spiramine C (yielding dihydroajaconine 8, confirming structure except for C-19 stereochemistry). Three-dimensional X-ray crystallography confirmed spiramine A’s complete structure, including the oxazolidine ring’s stereochemistry. Spiramine B (C24H33NO4, mp 129–131°C, [α]²⁵ᴰ -159.5°), epimeric at C-19 with A, epimerizes in polar solvents to form a 1:1 mixture of A and B, supporting its structural assignment. 13C NMR analysis revealed that spiramines C and D possess C-19 stereochemistries corresponding to spiramines A and B, respectively, leading to their structural assignments as 5 and 6.