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The superconductivity of alkali-metal intercalated BC2, MxBC2 (M = Li, Na, and K; x = 0.5–1.5), has been studied using first-principles calculations. The calculated critical temperature (Tc) values are substantially high at x = 0.5 (49.8–57.1 K), which are higher than those for MgB2 and close to those predicted for LixByCz compounds. The Tc values at x = 1.5 are comparatively low (0.6–5.6 K) and close to those for graphite intercalation compounds. No superconductivity is observed at x = 1.0 for all alkali metals. An analysis of the electronic structures reveals that at x = 0.5, the state at the Fermi energy includes the σ bond character. In contrast, at x = 1.5, the state includes only π bonds comprising pz orbitals of B and C atoms. The σ bond character is essential for attaining high Tc values because the σ bond couples strongly with the bending-like phonon modes of the BC2 layer. However, the π bond couples weakly with the stretching-like phonon modes due to the small overlap of the pz orbitals, which results in a relatively low Tc for the material.