We investigate the crystal structure, magnetic properties, and crystalline-electric field of tetragonal, $I4_1/amd$, NaCeO$_2$. In this compound, Ce$^{3+}$ ions form a tetragonally elongated diamond lattice coupled by antiferromagnetic interactions ($Theta_{CW} = -7.69$ K) that magnetically order below $T_N = 3.18$ K. The Ce$^{3+}$ $J = 5/2$ crystalline-electric field-split multiplet is studied via inelastic neutron scattering to parameterize a $J_{eff} = 1/2$ ground state doublet comprised of states possessing mixed $|m_z rangle$ character. Neutron powder diffraction data reveal the onset of $A$-type antiferromagnetism with $mu=0.57(2)$ $mu_B$ moments aligned along the $c$-axis. The magnetic structure is consistent with the expectations of a frustrated Heisenberg $J_1$-$J_2$ model on the elongated diamond lattice with effective exchange values $J_1 > 4 J_2$ and $J_1 > 0$.
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