Catalyzes the NADP(+)-dependent oxidation of succinate semialdehyde to succinate. Although it has succinate semialdehyde dehydrogenase activity, is likely to act physiologically on a different aldehyde(s) (By similarity). H2O + NADP(+) + succinate semialdehyde = 2 H(+) + NADPH + succinate Belongs to the aldehyde dehydrogenase family.