Invasive pentatomid Halyomorpha halys recently arrived to the Black Sea region and began damaging agricultural and ornamental plants. We studied the effects of day length and temperature on the pre-adult development and diapause induction in H. halys from Sochi (Russia) under laboratory conditions (20, 24, and 28 °C and several photoperiods). The pre-adult development of H. halys was noticeably faster under L:D 12:12 compared with L:D 15:9. The sum of effective temperatures required for the pre-adult development was ca. 530 and 590 degree days under these two conditions, respectively, whereas the lower developmental thresholds were similar (ca. 13.3 °C). Adults of H. halys demonstrated a typical long-day-type photoperiodic response of facultative winter adult diapause induction: Short days (photophases of 12–15 h) induced diapause in all adults, whereas long days (with photophases longer than 15 h) promoted reproduction. The photoperiodic responses of diapause induction of females and males were very similar. At 24 °C, the threshold of the response was between 15 and 16 h. At 20 °C, even under the very long-day conditions (L:D 18:6) about 50% of adults entered diapause. Field records suggest that H. halys likely produces two generations per year in Sochi. Short days might accelerate nymphal growth of the second generation in August and then induce winter diapause in adults. Phenological studies and monitoring are needed for a better understanding of the adaptation process of this invasive pest to new conditions.