Fe-based alloys, such as ferritic stainless steels, developed for planar solid oxide fuel cell (pSOFC) interconnect commonly contain Cr and can form a chromia (Cr₂O₃) scale on the surface. At the high operating temperature of pSOFC, Cr can be oxidized to volatile Cr⁺⁶ species which can deposit in the cathode and lead to degradation of cathode performance and limit the lifetime of a pSOFC system. Coating a suitable material, such as lanthanum strontium manganite (LSM), has been practically conducted on the metallic interconnect to prevent such Cr poisoning in the cathode side of pSOFC. In this study, the joint strength between an SOFC glass-ceramic sealant (GC-9) and an interconnect steel (Crofer 22 APU) coated with La_0.67Sr_0.33MnO₃ is investigated at room temperature (RT) and 800 °C in air under both shear and tensile loading modes. In particular, the effect of a long-term thermal aging at 800 °C on the joint strength is characterized. Results indicate that an LSM coating on the metallic interconnect degrades the joint strength between the given GC-9 glass-ceramic sealant and Crofer 22 APU interconnect steel. The shear strength at RT and 800 °C is significantly degraded by 36% and 65%, respectively, and the tensile strength is significantly reduced by 39% and 80% at RT and 800 °C, respectively. It is attributable to the existence of pores around the interface between GC-9 sealant and LSM coating. However, the shear strength of LSM-coated joint specimen is enhanced by 52% at RT and by 200% at 800 °C after 1000-h thermal aging in air. The given thermal aging treatment also improves the tensile strength of LSM-coated joint specimen by 50% at 800 °C. This may be attributed to a self-healing effect of the GC-9 glass-ceramic during thermal aging to reduce the pore size. Fractography analyses reveal that fracture path involving the LSM layer and its adjoining interfaces accompanies a lower joint strength.