The effect of using drastically reduced growth rates on the surface morphology of YBa2Cu3Oy (YBCO) thin films grown on lattice‐matched (110) NdGaO3 substrates using the reactive coevaporation technique in a radical oxygen atmosphere is reported. The surface of films grown at an ultralow growth rate (∼0.0065 nm/s) appears to be very smooth without any precipitates, holes, crevasses, or pits. The film surface roughness is on the order of the unit‐cell constant of superconducting YBCO, about 1.2 nm. This shows that decreasing the growth rate enhances the lateral spreading of nucleated islands, with a height of one unit cell, in the Stranski–Krastanov growth mode [Ber. Akad. Wiss, Wien 146, 797 (1938)].