This study investigates the electronic structure of Sr₂IrO₄ using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements, along with first-principles band calculations. The system is found to be well described by novel effective total angular momentum J_eff states, where relativistic spin-orbit (SO) coupling is fully considered under a large crystal field. Despite delocalized Ir 5d states, the J_eff states form narrow bands, leading to a J_eff = 1/2 Mott ground state with unique electronic and magnetic properties. This suggests a new class of quantum spin-driven correlated-electron phenomena. The J_eff = 1/2 Mott state arises due to strong SO coupling, which splits the 5d states into effective total angular momentum J_eff = 1/2 and 3/2 bands. The system is effectively reduced to a half-filled J_eff = 1/2 single band system, leading to a narrow band with a small correlation energy that opens a Mott gap. The narrow band width is due to reduced hopping elements of the J_eff = 1/2 states with isotropic orbital and mixed spin characters. First-principles band calculations confirm the formation of J_eff bands due to large SO coupling. ARPES results support the J_eff = 1/2 Mott state, showing dispersive band features and valence band maxima topology consistent with the LDA + SO + U results. The optical conductivity shows an insulating gap and a double-peak feature, consistent with the J_eff = 1/2 Mott state. XAS results confirm an orbital ratio of 1:1:1 for the unoccupied t₂g state, validating the J_eff = 1/2 state. The J_eff = 1/2 Mott state exhibits unusual magnetic behavior, with the total magnetic moment dominated by the orbital moment. The magnetic ground state has weak ferromagnetism due to canted antiferromagnetic order with a 22° canting angle. The local moment at each Ir site is 0.36 μB, with 0.10 μB spin and 0.26 μB orbital contributions. This value is about one-third of the ionic value but retains the 1:2 ratio. The study highlights the importance of strong SO coupling in 5d transition-metal oxides, leading to a new class of materials with spin-orbit integrated narrow band systems. The findings suggest that the underlying physics of 5d TMOs is not a simple continuation of 3d TMO physics and requires a new paradigm for understanding their unique phenomena.This study investigates the electronic structure of Sr₂IrO₄ using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements, along with first-principles band calculations. The system is found to be well described by novel effective total angular momentum J_eff states, where relativistic spin-orbit (SO) coupling is fully considered under a large crystal field. Despite delocalized Ir 5d states, the J_eff states form narrow bands, leading to a J_eff = 1/2 Mott ground state with unique electronic and magnetic properties. This suggests a new class of quantum spin-driven correlated-electron phenomena. The J_eff = 1/2 Mott state arises due to strong SO coupling, which splits the 5d states into effective total angular momentum J_eff = 1/2 and 3/2 bands. The system is effectively reduced to a half-filled J_eff = 1/2 single band system, leading to a narrow band with a small correlation energy that opens a Mott gap. The narrow band width is due to reduced hopping elements of the J_eff = 1/2 states with isotropic orbital and mixed spin characters. First-principles band calculations confirm the formation of J_eff bands due to large SO coupling. ARPES results support the J_eff = 1/2 Mott state, showing dispersive band features and valence band maxima topology consistent with the LDA + SO + U results. The optical conductivity shows an insulating gap and a double-peak feature, consistent with the J_eff = 1/2 Mott state. XAS results confirm an orbital ratio of 1:1:1 for the unoccupied t₂g state, validating the J_eff = 1/2 state. The J_eff = 1/2 Mott state exhibits unusual magnetic behavior, with the total magnetic moment dominated by the orbital moment. The magnetic ground state has weak ferromagnetism due to canted antiferromagnetic order with a 22° canting angle. The local moment at each Ir site is 0.36 μB, with 0.10 μB spin and 0.26 μB orbital contributions. This value is about one-third of the ionic value but retains the 1:2 ratio. The study highlights the importance of strong SO coupling in 5d transition-metal oxides, leading to a new class of materials with spin-orbit integrated narrow band systems. The findings suggest that the underlying physics of 5d TMOs is not a simple continuation of 3d TMO physics and requires a new paradigm for understanding their unique phenomena.