Three‐dimensional analysis of a heat‐spreading phenomenon in a heat sink of phase‐locked arrays of oxide‐isolated diode lasers is carried out in the present paper. The main result is that with increasing number of laser elements the heat‐sink temperature increase turns out to change sublinearly whereas temperature differences between adjacent edge lasers increase considerably which threatens stability of the phase‐locking operation. The heat spreading in semiconductor, contact, and solder layers was approximated by a one‐dimensional model. The special correction factor (b) accounting for the two dimensionality of this heat spreading was used to improve the accuracy of the calculations.