The aim of this study was to determine the effect of a mycorrhizal inoculation (AMF) and irrigation regime on certain yield morphological parameters and the biological value of fruits of open field-grown sweet pepper under temperate climate conditions. A study on the Polish hybrid cultivar ‘Roberta F1’ was conducted over the period 2016–2018 in a private certified organic farm. Sweet pepper was harvested at physiological maturity from the second 10 days of August to the first 10 days of October. AMF and irrigation were shown to significantly modify the selected morphological parameters of the peppers. Fruits with the highest weight, length, and width were harvested from AMF-inoculated plants, both irrigated and non-irrigated ones. The chemical composition and antioxidant activity (AA) of pepper fruit extracts were significantly affected by AMF and irrigation. AMF application contributed to a decrease in the percentage of dry matter, vitamin C, reducing sugars, extract, carotenoids, and AA. Irrigation, on the other hand, had a beneficial effect on enhancing the biological value of pepper fruits (except for vitamin C), also increasing their AA. The highest levels of carotenoids (4.64 mg 100 g−1 of fresh matter (FM) were found in the fruits of irrigated plants without AMF, whereas the highest levels of vitamin C (134.10 mg 100 g−1 FM) were accumulated by the fruits of plants grown without AMF and without irrigation.