The photoperiod alone or in combination with temperature may affect macroalgal growth, reproduction, and development. This study aimed to investigate the responses of Ericaria barbatula , a perennial fucalean species collected from Kavala Gulf, Greece, to photoperiod and temperature. Maximum quantum yield ( F v / F m ), wet biomass ( g ), relative growth rate (day −1 ), and reproduction (receptacles and zygote formation) were monitored in two laboratory experiments, the first one in two consecutive phases (A and B), that lasted at least seven months. While in Experiment 1, phase A, specimens were cultivated under long-day (LD) and short-day (SD) conditions at a constant temperature, in Experiment 1, phase B, they were cultivated at a seasonally changing temperature. The results indicated that E. barbatula reproduced in both phases under SD conditions and grew better in winter to early spring under seasonally changing temperatures. The cultivation under night-break (NB) daylength and seasonally changing temperatures (Experiment 2) indicated that E. barbatula is a facultative (quantitative) SD plant, a result that needs further examination. However, the induction of reproductive organs under SD conditions has been confirmed. It secures the release of zygotes and the germlings’ growth in the field from April to June/July under non-stressful Mediterranean Sea temperatures.