Climate change is presumed to increase both the number and frequency of fluctuations in environmental conditions. Fluctuations can affect the ecological and evolutionary processes that make species more successful competitors against other species. For example, fluctuating conditions can create selection pressures for traits that are profitable in adaptation to fast climate change. On an ecological timescale, environmental fluctuations can facilitate species competitive success by causing reductions in other species’ population sizes. Climate change could then enhance species invasions into new areas if fluctuation-adapted invaders displace their native competitors in chancing environments. We tested experimentally whether fast environmental fluctuations, either past (on an evolutionary timescale) or present (on an ecological timescale) affect species competitive success. Bacteria evolved in either constant or fluctuating temperature were competed against the dominant invader Serratia marcescens, which had also evolved in either constant or fluctuating temperature. Moreover, the competition experiments were conducted in environments with similarly constant or fluctuating thermal conditions. The results showed that temperature fluctuations during competition, i.e. on an ecological timescale, made the invader more successful. Surprisingly, we did not find that the invaders’ or its competitor species’ evolution in fluctuating environments would have influenced the outcome of the competition. Our study highlights the importance of the present environmental fluctuations in promoting species’ competitive success and potentially facilitating biological invasions.