This paper reports on a new analysis of archival ALMA 870 μm dust continuum observations. Along with the previously observed bright inner ring (r ~ 20–40 au), two addition substructures are evident in the new continuum image: a wide dust gap, r ~ 40–150 au, and a faint outer ring ranging from r ~ 150 au to r ~ 250 au and whose presence was formerly postulated in low-angular-resolution ALMA cycle 0 observations but never before observed. Notably, the dust emission of the outer ring is not homogeneous, and it shows two prominent azimuthal asymmetries that resemble an eccentric ring with eccentricity e = 0.07. The characteristic double-ring dust structure of HD 100546 is likely produced by the interaction of the disk with multiple giant protoplanets. This paper includes new smoothed-particle-hydrodynamic simulations with two giant protoplanets, one inside of the inner dust cavity and one in the dust gap. The simulations qualitatively reproduce the observations, and the final masses and orbital distances of the two planets in the simulations are 3.1 M_J at 15 au and 8.5 M_J at 110 au, respectively. The massive outer protoplanet substantially perturbs the disk surface density distribution and gas dynamics, producing multiple spiral arms both inward and outward of its orbit. This can explain the observed perturbed gas dynamics inward of 100 au as revealed by ALMA observations of CO. Finally, the reduced dust surface density in the ~40–150 au dust gap can nicely clarify the origin of the previously detected H₂O gas and ice emission.