We present a derivation and experimental implementation of a dimension-dependent contextuality inequality to certify both the quantumness and dimensionality of a given system. Existing methods for certification of the dimension of a quantum system can be cheated by using larger classical systems, creating a potential loophole in these benchmarks, or can in practice only be evaluated assuming pure quantum states. Our approach uses contextuality inequalities that cannot be violated by classical systems thus closing the previous loopholes. We validate this framework experimentally with photons, observing violations of a CHSH-based contextuality inequality and surpassing the qutrit bound of the CGLMP₄-based contextuality inequality. These show that contextuality can be used for noise-robust tests of the number of qubits.