We propose an altermagnetism-induced route to a two-dimensional higher-order topological Dirac semimetal in a trilayer nonsymmorphic Dirac system. Interlayer hybridization preserves the nonsymmorphic bulk Dirac crossings and generates helical edge states in the trilayer phase. Introducing outer-layer $d$-wave altermagnetic order gaps the helical edge states, while only the symmetry-protected Dirac crossing at the high-symmetry point $M$ remains gapless. The resulting phase hosts a surviving bulk Dirac point, gapped edge spectra, and corner states. An exact layer-space decomposition further reveals that these features arise from the coexistence of a gapless Dirac sector and a gapped ${\mathbb{Z}}_2$ topological sector. Our results establish altermagnetism as an effective mechanism for engineering higher-order topological Dirac semimetals in two-dimensional nonsymmorphic systems.

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• Altermagnetism
• Topological edge modes
• 2-dimensional systems
• Dirac semimetal
• Topological materials
• Tight-binding model