Altermagnets with nonrelativistic momentum-dependent spin splitting and compensated net magnetic moments have recently garnered significant interest in spintronics, particularly as pinning layers in magnetic tunnel junctions (MTJs). However, room temperature (RT) altermagnet-based MTJs with tunable tunneling magnetoresistance (TMR) or electroresistance (TER) modulated by multiferroicity remain largely unexplored. Here, we propose an experimentally fabricable above-RT multiferroic MTJ, comprising an altermagnetic metal, ferroelectric barrier, and ferromagnetic metal–epitomized by a $\mathrm{Cr}\mathrm{Sb}\text{/}{\mathrm{In}}_2{\mathrm{Se}}_3\text{/}{\mathrm{Fe}}_3\mathrm{Ga}{\mathrm{Te}}_2$ heterostructure. Our calculations with first-principles and nonequilibrium Green's function method indicate that the architecture enables magnetically switchable TER, electrically tunable TMR, and dual-mode controllable spin filtering. To disentangle the roles of ferroelectricity and the tunnel barrier, nonferroelectric $\mathrm{S}{\mathrm{b}}_2{\mathrm{Se}}_3$ and a vacuum gap are exploited as control cases. Remarkably, the system achieves TMR up to 2308%, TER of 707%, and near-perfect spin filtering efficiency. Both TMR and TER are considerable for $\mathrm{Cr}\mathrm{Sb}\text{/}{\mathrm{In}}_2{\mathrm{Se}}_3\text{/}{\mathrm{Fe}}_3\mathrm{Ga}{\mathrm{Te}}_2$ with either Cr or Sb interface. The transport performance is robust under bias voltage. These findings demonstrate the above-RT multiferroic altermagnet-based MTJs and highlight their exciting potential as a versatile platform for next-generation spin dynamics, magnetic sensing, and quantum logic nanodevices.

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• Magnetism
• Spintronics
• Altermagnets
• Multiferroics
• Tunnel junctions
• First-principles calculations