The lumpfish (Cyclopterus lumpus L.) has recently become relevant for aquaculture due to its appetite for the ectoparasitic copepod, the sea louse (Lepeophtherius salmonis Krøyer). In order to determine the effect of temperature on growth properties of juvenile lumpfish, two size groups (6.2 g, S.E. ± 0.1 and 26.5 g, S.E. ± 0.6) were compared in terms of temperature- and size-related growth at 4, 7, 10, 13, 16 °C and varying ambient temperature. The overall highest growth rates were observed for fish < 120 g at 13 and 16 °C (3.65 and 3.60% day^− 1), and declined stepwise with decreasing temperature. Only a modest response to temperature was observed for large fish > 120 g. Size rank correlations increased with increasing body weight. The results presented in this study suggest that optimum temperatures for growth (T_optSGR) decreased with increasing fish size from 15.7 °C for 11.0–20.0 g fish, 16.1 °C for 20.0–40.0 g fish, 13.1 °C for 100.0–110.0 g fish, to 8.9 °C for 120.0–200.0 g fish.