In the last years several countries suffered from unusual heat waves in summer. High temperature and the related heat stress (HS) diminish the productivity and reproductivity of dairy cows. The aim of this study is to compare the productive and reproductive parameters in dairy cows (n = 250 Holstein cows) supplemented with a phytogenic feed additive (Herb-All^TM COOL in a dose of 12 g/cow/day) with the non-supplemented group (control) (n = 250 Holstein cows) in hot seasons. In the present trial, phytogenic feed additive (PFA) supplementation restored the animals' normal physiological feeding and rumination patterns in Italy (the temperature humidity index “THI" ranged between 77 and 81). It enabled a significant increase in milk production (P ≤ 0.05). This beneficial effect could even last two months after the supplementation was discontinued. In spite of the increase in milk volume, the concentration of the main milk components (milk fat and proteins) remained unchanged. The cows supplemented with PFA were healthier and expressed fewer postpartum complications, requiring fewer medications (-28.5%) and reducing the need to cull high-producing cows later (-40.3%) (P ≤ 0.05). Furthermore, PFA enabled significant improvements in fertility parameters (the period to confirmed pregnancy was reduced by 21.3%, the success rate at first insemination was more than doubled (+106.9%), and the total success rate after insemination was improved by 41.4%, where more than 68% of the cows were successfully inseminated) (P ≤ 0.05). In conclusion, PFA supplementation of dairy cows was capable of restoring both reproductive and lactational potential to thermoneutral levels.