Quantum phase transitions (QPTs) describe when a many-body quantum system displays non-analytic behavior associated with a discontinuous change in a property of the ground state as a parameter is varied. The QPT in prototypical Dicke model is difficult to reach experimentally as the spin–field coupling strength must be quite large. In this work we describe a new model—the off-resonant Tavis–Cummings model where we drive the common mode, and discover a new type of QPT at quite low coupling strengths which are comparable with the geometric mean of the atomic and field detunings . Through analytic methods we demonstrate this QPT for both finite and infinite numbers of spins and show that |〈Jx(Jz)〉|/(N/2) ∼ |λ/λc − 1|γx(γz) and 〈a†a〉/N ∼ |λ/λc − 1|γa for λ ⩾ λc, with critical exponents γx ≈ 1/2, γz ≈ 1 and γa ≈ 1. We show that this QPT can be immediately observed by laboratory cavity-QED setups such as Bose–Einstein condensate in optical cavity and superconducting circuit-QED as well as a line of trapped ultracold ions.