Altermagnetism as a new concept in condensed matter physics is currently being thoroughly investigated. Despite the absence of macroscopic magnetization, altermagnets host a hidden spin order whose direct detection remains a challenge. Here we report on the observation of electric and magnetic dipole forbidden optical second-harmonic generation (SHG) in the altermagnet CoF₂ with a centrosymmetric lattice and spin order. We demonstrate that below the Néel temperature T_N = 38 K the SHG signal is sensitive to the ferrotype magnetic octupole O^M which is the order parameter in the antiferromagnetic phase. By combining polarization-resolved SHG experimental data and a phenomenological symmetry analysis, we show that the altermagnetic spin structure of CoF₂ enables a ferroic-octupole-induced electric-quadrupolar nonlinear polarization P^(2ω) = iε₀ ᶜχ⁽³⁾(O^M) :E^ω ∇ E^ω. The temperature dependence of SHG reveals a phase transition at T_N confirming the spin origin of the observed signal. The SHG response is resonantly enhanced by a coherent three-photon process caused by the electronic d-d transitions of the Co²⁺ ion. Model calculations of SHG polarization rotational anisotropies and temperature dependencies give a reasonable agreement with experimental data, proving the disclosed nonlinear contribution. Our results establish SHG as a novel sensitive tool of ferroic-octupolar spin ordering and highlight the potential of CoF₂ and other altermagnets for further nonlinear optical investigations and applications.