Permanently magnetic Ganymede carves a distinct magnetosphere inside Jupiter’s larger magnetic domain, confined by ambient Jovian plasma and magnetic field along the upstream magnetopause. As Ganymede traverses the Jovian plasma sheet, ambient magnetoplasma conditions vary at half-synodic period (5.27 hr), leading to same-period oscillation in the Chapman-Ferraro (C-F) magnetic field produced by the magnetopause. We assess the C-F magnetic field as a unique excitation source for Ganymede’s subsurface ocean. The magnetopause field is shown to diffuse into but not through the ocean, causing magnetic induction provided the liquid layer thickness is >~30 km. Then using an analytical model, we calculate maximum variation of the C-F field and estimate ~1-10 nT inductive response from the ocean subjected to the magnetopause’s movement, a range detectable by the JUpiter ICy moon Explorer (JUICE) spacecraft. Hence, Ganymede’s magnetopause may become a viable tool for future induction-based study of the moon’s subsurface ocean.