This two-dimensional numerical study investigates flow-induced vibration (FIV) and heat transfer in 3 × 3 arrays of nine heated circular cylinders of equal and unequal diameter configurations at a constant Re = 100. Six out of the nine cylinders are stationary, while three are elastically supported in the vertical direction. The computations are carried out using the commercial software ANSYS Fluent loaded with user-defined function (UDF). The effects of transverse spacing ratio (G* = 2, 3, and 4), diameter (D) and reduced velocity (2 ≤ Ur ≤ 20) on time histories of cylinder displacements, vorticity contours, oscillation amplitudes, mean pressure coefficient and Nusselt number (Nu) are investigated by considering forced convection heat transfer. Results show that lower oscillation amplitudes of elastically supported cylinders are noticed in arrays of cylinders when all cylinders are placed closely, i.e., at G* = 2. Furthermore, as G* increases, the oscillation amplitudes also increase due to the formation of vortex shedding. A better heat transfer performance is observed for arrays of cylinders with varying diameters at G* = 2 compared to those with the same diameter. For G* = 4, arrays of cylinders with the same diameter possess better heat transfer for all Ur. FIVs affect the Nu of elastically supported cylinders as well as stationary cylinders.