Topological superconductivity hosts protected quasiparticles and is central to topological quantum computation, yet its realization in intrinsic materials remains challenging and often relies on engineered platforms. Here we uncover a symmetry-constrained mechanism for nodal topological superconductivity in altermagnets. Focusing on fourfold rotational collinear altermagnets, we show that the native crystalline antiunitary symmetry TC_4z generically forbids pure spin-singlet pairing and selects pairing structures that admit Bogoliubov-de Gennes (BdG) Hamiltonians with emergent chiral symmetries. These symmetries further give rise to robust nodal topological phases over broad parameter regimes, including a nodal-point phase hosting Majorana flat bands (MFBs) and two distinct nodal-loop phases with chiral Majorana edge states. Notably, the nodal structure persists even after spontaneous breaking of the antiunitary symmetry, indicating that the topology originates from symmetry-constrained pairing rather than direct symmetry protection. Finally, we propose tunneling signatures that can distinguish these nodal phases and probe symmetry breaking experimentally.