We theoretically investigate spin-dependent transport in altermagnet/triplet-superconductor/altermagnet (AM/TSC/AM) junctions in the presence of interfacial Rashba spin-orbit coupling (RSOC). Within a microscopic Bogoliubov–de Gennes scattering formalism, we compute angle- and energy-resolved conductance, spin polarization, zero-bias response, and tunneling magnetoresistance (TMR) for nodal $p_x$ and chiral $p_x+i{p}_y$ superconductors. Although altermagnets carry no net magnetization, the momentum-dependent spin splitting, combined with RSOC enables a pronounced spin valve effect without ferromagnetic electrodes. We show that conductance, spin polarization, and TMR exhibit distinct and robust fingerprints of the triplet pairing symmetry. For nodal $p_x$ superconductor, sign-change-induced surface Andreev bound states dominate subgap transport, producing strongly anisotropic conductance, giant zero-bias spin polarization, and a monotonic enhancement of TMR with increasing RSOC. In contrast, the chiral $p_x+i{p}_y$ state exhibits smoother conductance and polarization profiles governed by topological edge modes, resulting in broader, lobelike TMR patterns with weaker sensitivity to interface transparency. Moreover, RSOC can acts as an electrically tunable spin-mixing knob, while the barrier strength controls coherence and energy selectivity, together enabling large, symmetry controlled spin filtering and magnetoresistance. Our results establish AM/TSC/AM junctions as a symmetry sensitive transport platform for realizing electrically tunable spin valve functionality and probing triplet pairing without ferromagnetic components.

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• Andreev bound states
• Rashba coupling
• Spin polarization
• Spin-triplet pairing
• Tunneling magnetoresistance
• Altermagnets
• Unconventional superconductors
• Bogoliubov-de Gennes equations
• S-matrix method in transport