We measured the density of iron-ringwoodite and its pressure and temperature dependence at conditions of the mantle transition zone using the laser-heated diamond anvil cell in conjunction with X-ray diffraction. Our new data combined with previous measurements constrain the thermoelastic properties of ringwoodite as a function of pressure and temperature throughout the transition zone. Our best fit Mie-Grüneisen-Debye equation of state parameters for Fe end-member ringwoodite are V0 = 42.03 cm3/mol, K0 = 202 (4) GPa, K′ = 4, γ0 = 1.08 (6), q = 2, and θD = 685 K. This new equation of state revises calculated densities of the Fe end-member at transition zone conditions upwards by ∼0.6% compared with previous formulations. We combine our data with equation of state parameters across the Mg-Fe compositional range to quantify the effect of iron and temperature on the density and bulk sound velocity of ringwoodite at pressure and temperature conditions of the Earth's transition zone. The results show that variations in iron content and temperature have opposing effects on density and bulk sound velocity, suggesting that compositional (iron content) and temperature variations in the transition zone may be distinguished using seismic observables.